Geographic distance from Europe helped secure the United States from foreign invasion, but it could not protect against internal rupture. The Civil War was the first American conflict since the Revolution with truly existential stakes. It resulted in new constitutional interpretations both Congress and the President now considered essential for the Republic’s survival.

The South’s violent rebellion and the Union’s massive campaign to quash it also unleashed a new type of warfare at a colossal scale, enabled by technologies like railroads and mass-production factories. The national government expanded, mobilized, and improvised – its powers growing with the war itself.

Once the national government asserted power to resist state secession by force, the central constitutional question changed: no longer whether to make war, but how to wage it – and by whose authority. Throughout the conflict, war powers and war strategy fortified one another. Both Congress and, at times, Lincoln acting alone defended what would have been constitutionally unthinkable in peacetime as permissible in war. The justification was military necessity – nowhere invoked more dramatically than in the struggle over slavery. The Union’s expansive use of war powers in this unprecedented conflict then raised, without resolving, new legal questions about when war ends – and with it the reach of war powers.

When national self-preservation displaced constitutional theories of limited power, politics became the main brake on war powers. Lincoln and his fellow Republicans in Congress could only go as far as a restive public and Union solidarity would permit. Nonetheless, law remained central to the debate – perhaps not always directly restricting the government’s actions, but shaping the landscape on which various actors tested arguments and sold them to the public. Even the fiercest war-strategy disputes featured rival views of how the Constitution divides and checks power for starting, waging, and ending armed conflicts. And whereas one might expect that such a catastrophic and singular conflict would today be considered legally aberrant, the Civil War yielded precedents that lawyers today consider canonical. Given the Civil War’s carnage and its shattering of the Union, the surprise was not the scarcity of legal debate, but its abundance.

Emergency Military Action

The first and most urgent question was how far the President could act when rebellion erupted and Congress was away. In 1860, Abraham Lincoln won the presidency on a platform opposing the spread of slavery into new U.S. territories. Long-standing sectional tensions over that issue reached their breaking point with his election. Shortly thereafter, South Carolina seceded, and six other Southern slave states quickly followed to form the independent Confederacy. Lincoln’s inaugural address in March 1861 offered conciliation; the Confederacy refused it. The Civil War detonated that April, when South Carolina’s militia attacked Union forces stationed at Fort Sumter in Charleston Harbor. For the next four years, the United States fought what would become the bloodiest war in its history.¹

In the conflict’s first three months, the President undertook a series of emergency actions – some unilateral, controversial, and possibly unconstitutional – that likely averted the republic’s collapse. Congress was in recess when Fort Sumter fell, its members scattered weeks away. Rather than immediately summon them back for an emergency session to request new legislative authority, Lincoln delayed and acted alone. Emergency action saved the nation – but it strained its very legal foundations.

Some of his moves rested on statutory foundations; others did not. The President had standing authority from Congress to call out the state militias, under the Militia Acts from the 1790s, whose modern successors are discussed in Chapter 5. An 1807 statute – the Insurrection Act, likewise still on the books in amended form today – also authorized him to use the regular armed forces and state militias to put down rebellions. On April 15, 1861, he called up 75,000 men from Northern states under authority delegated to him by those statutes.

Judging them insufficient, Lincoln expanded and armed the Union’s land and naval forces beyond the numbers set by Congress. He also ordered a blockade of Southern ports to cut trade and cripple Confederate military supply. And he authorized in certain areas the suspension of habeas corpus, the ancient and constitutionally protected right to challenge the legality of one’s imprisonment.

Some of Lincoln’s opening moves skirted the Constitution’s edges. The most remarkable came in May 1861, when he called for extra volunteers to enlarge the Army and Navy. The Framers had given Congress that power for a reason: to guard against military tyranny. Here was the President bypassing a fundamental check, though he pledged to put the issue before Congress when it reassembled.² Despite Congress’s power of the purse, Lincoln also took $2 million from the Treasury to buy war materiel through private citizens rather than government employees (many of whom he distrusted as harboring Confederate sympathies).

There was some limited precedent for this type of action – by Thomas Jefferson, no less.³ In 1807, a British warship patrolling the Virginia coast attacked an American frigate that the British suspected to be harboring deserters. With Congress in recess, Jefferson embargoed British ships, branded their armed vessels enemies, and contracted for more naval forces.⁴ He did not believe that existing statutes authorized these actions; instead, he expected that Congress would ratify the emergency actions after the fact (which it did). In asking Congress to do so, Jefferson stressed that “[t]o have awaited a previous and special sanction by law would have lost occasions which might not have been retrieved.”⁵

Lincoln’s decision to expand the Army and Navy was on a much larger scale, though – and so were the stakes. In his case, he sidestepped the Constitution to save it.

With Congress still on recess – because Lincoln had declined to call it back right away – the President finally convened a special legislative session beginning symbolically on July 4, 1861. In a lengthy address, he offered Congress a detailed justification for his many actions – which at this point were faits accomplis. Lincoln characterized the situation as a self-defensive war of survival: “no choice was left but to call out the war power of the Government and so to resist force employed for its destruction by force for its preservation.”⁶ In so doing, he defended some of his specific moves like the mobilization and deployment of militias and the blockade of Southern ports as “strictly legal.” The former move rested firmly on the Militia Acts; the latter would prove more complicated.

Then, much like Jefferson had done in the Chesapeake affair, Lincoln appealed to Congress to approve after-the-fact his emergency military expansion: “These measures, whether strictly legal or not, were ventured upon under what appeared to be a popular demand and a public necessity, trusting then, as now, that Congress would readily ratify them.”⁷ The President came close to conceding the move was unlawful – that it rested on neither authority delegated by Congress nor his own constitutional power – counting on Congress to later excuse and approve the brief, justified encroachment on its power.

For the most part, Congress obliged. It passed legislation authorizing some of Lincoln’s specific moves, and in August, it passed a general resolution declaring many of his extraordinary military actions “hereby approved and in all respects legalized and made valid, to the same intent and with the same effect as if they had been issued and done under the previous express authority and direction of the Congress of the United States.”⁸ Like the President’s ambiguous stance on the legality of his actions, Congress left unclear whether it regarded legislative approval to be essential – and thus whether some moves without it would be unconstitutional. As explained in the next section, Congress’s response to Lincoln’s suspension of habeas corpus was a special case.

That Lincoln would later emerge as one of the nation’s greatest and most influential wartime presidents is surprising on many levels. He was among the least militarily experienced presidents to date, having served only briefly and without distinction in the Illinois militia. By contrast, Confederacy President Jefferson Davis was a West Point graduate, veteran Army officer, and former secretary of war.⁹ Furthermore, Lincoln moved cautiously in establishing precedents; he was concerned that his wartime clout not become routine presidential power.¹⁰ He did what he thought was necessary to save the nation in a true war of survival, but he likely would have opposed future presidents citing his actions to justify broad executive power in situations less dire. As he put it in the July 4 address, “It was with the deepest regret that the Executive found the duty of employing the war power in defense of the Government forced upon him. He could but perform this duty or surrender the existence of the Government.”¹¹

If one urgent challenge was raising Union forces, another was getting them to the front – an imperative that soon collided with individual rights and access to the courts.

Suspending Habeas Corpus

Of all the unilateral moves Lincoln defended in that July 4 address, his suspension of habeas corpus was and still remains the most controversial.¹² The May 1861 case of a Marylander named John Merryman produced a dramatic legal showdown over that issue between the President and the judiciary – a showdown in which Lincoln defied the Chief Justice after losing in court.

The case, Ex parte Merryman, begins as a story about railroads and geography. Emerging technology – superior firepower in new weapons, faster communications via telegraph, and greater mobility by train – was transforming nineteenth-century warfare. In a contest of supply, the North’s dense railway network far outmatched the South’s sparse lines. In the chaotic opening phase of the conflict, though, tracks feeding the Union capital were a vulnerable chokepoint.

The capital city of Washington is tucked between Maryland and Virginia. Virginia was Confederate; the population of Maryland, a slave state on the fence, leaned pro-South. So, when Union soldiers en route by train to Washington were attacked in Baltimore by thousands of Confederate supporters, Maryland’s governor and a delegation of Baltimore politicians (many of them Southern-leaning) warned Lincoln not to send more Union soldiers to the state. The President went ahead anyway, explaining “[o]ur men are not moles, and can’t dig under the earth; they are not birds, and can’t fly through the air. There is no way but to march across [Maryland], and that they must do.”¹³ Amid fears of further violence, Maryland state and local officials began to order the destruction of the city’s bridges and telegraph lines connecting the state – and the nation’s capital – to the North.

To keep open the critical corridor, Lincoln wrote in April to General Winfield Scott, the Commanding General of the U.S. Army, authorizing Scott and his officers to suspend habeas corpus – and thereby detain insurrectionists to prevent guerrilla activities along that military transit route. Lincoln knew that suspending habeas corpus was a grave step. He described it to General Scott as a more extreme one than bombarding insurrectionist strongholds in the area, yet he deemed it necessary.¹⁴

Scott quickly activated those instructions. Early in the morning on May 25, his troops seized John Merryman from his home and carried him off to nearby Fort McHenry. A Maryland farmer, Merryman was also a pro-secessionist who had sabotaged train and telegraph lines to impede Union military operations. Within hours of his arrest, Merryman’s attorney drafted a habeas corpus petition – a petition that landed in the hands of Chief Justice Roger B. Taney, who was also (per judicial practice at the time) the designated judge for cases in Maryland.

Taney’s Confederate sympathies were well known. A native Marylander himself, the eighty-four-year-old had sat on the Supreme Court as Chief Justice since 1836. He was most closely associated with his infamous 1857 decision in Dred Scott v. Sandford, in which he declared that Black people, whether free or slave, had no standing as citizens under the Constitution.¹⁵

Taney now ruled that Merryman be released. Sharply rebuking Lincoln, Taney read the Constitution to give Congress – and only Congress – limited power to suspend habeas corpus. The Constitution’s drafters wrote the relevant clause in an awkward, passive voice: “The Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it.” But suspended by whom? Taney noted that the Clause appears in Article I, which enumerates Congress’s powers, and that the Constitution’s drafters feared granting such power to the President would invite tyranny.¹⁶

When Taney published his May 28, 1861, opinion, there was nothing he could do to enforce it, and he knew that.¹⁷ He concluded the opinion with instructions directing the clerk to send a copy to the President, noting: “It will then remain for that high officer, in fulfilment of his constitutional obligation to ‘take care that the laws be faithfully executed,’ to determine what measures he will take to cause the civil process of the United States to be respected and enforced.”¹⁸ Newspapers around the country reprinted Taney’s opinion, and Lincoln’s opponents used the case to paint him as a despot. His supporters denounced Taney for judicial overreach.¹⁹

Lincoln’s response to Taney’s judicial ruling is remarkable because it is so rare in presidential history: He refused to follow it. He then took his case to Congress when it reconvened on July 4. The President’s defense of his habeas corpus suspension and Congress’s response were more complex than those of his other unilateral actions.²⁰

Lincoln justified his move on two grounds. First, he claimed necessity, arguing that his duties to ensure that all laws be faithfully executed could only be fulfilled by suspending habeas corpus. “To state the question more directly,” he asked, “Are all the laws but one to go unexecuted, and the Government itself go to pieces lest that one be violated?” Second, he argued that the Constitution should be interpreted pragmatically to give the President power to suspend habeas corpus in crisis situations like this: “As the provision was plainly made for a dangerous emergency, it can not be believed the framers of the instrument intended that in every case the danger should run its course until Congress could be called together, the very assembling of which might be prevented, as was intended in this case, by the rebellion.”²¹

Congress took its time in responding to these arguments, and it declined to bless them outright – illustrating that in wartime Congress can often duck tough issues if the President acts on his own. A full year and a half later, Congress finally passed the Habeas Corpus Act. Going forward, that statute delegated to the President authority to suspend habeas corpus, subject to specific procedural requirements. The Act retroactively immunized officials for detentions made under Lincoln’s directives.²² The statute’s language is vague – probably deliberately so – as to whether it was endorsing the President’s military detentions to date or merely providing that no one should be punished for them. Congress also left ambiguous whether it regarded Lincoln’s unilateral suspension as intruding on its exclusive constitutional turf. Each branch could now claim vindication.

In any event, Lincoln, now backed by statute, imposed a sweeping suspension of habeas corpus.²³ In September 1863, he proclaimed that in his judgment “the public safety does require that the privilege of the said writ shall now be suspended throughout the United States” in cases where Union military forces “hold persons under their command or in their custody, either as prisoners of war, spies, or aiders or abettors of the enemy….”²⁴ Thousands of civilians – in the North, in border states, and in the Confederate South – were detained by military forces without trial.²⁵

Any constitutional democracy faces the dilemma of how to act when its survival is at stake and normal rules may not ensure it. Even when he was not following the letter of the Constitution, and despite the danger of his logic in the hands of an unscrupulous leader, Lincoln persuasively framed his positions in terms of high constitutional principles. Except for some secret moves to equip an expanded national army, he paired his bold assertions of unilateral presidential power with reasoned appeals to Congress for blessing after-the-fact. As with so many of Lincoln’s moves, that formula was both ethically and strategically deft, addressing immediate military needs while attentive to longer-term war politics shaped by law.

The Prize Cases: Resisting Force by Force

If detaining insurrectionists tested executive power on land, the effort to seal off Confederate ports did so at sea. Of all Supreme Court precedents from the war, none is more frequently invoked by modern executive branch lawyers than the case arising from that naval campaign, often read to endorse expansive presidential authority to use force in response to attack. There is some irony here, given that, as noted in Chapter 3, Lincoln had so sharply criticized the way Polk took the nation to war with Mexico.

Lincoln’s emergency actions soon after Fort Sumter had included proclaiming a blockade of Southern ports.²⁶ The agrarian South’s economy – and its military capacity – depended on the export of crops and imports of manufactured goods. The blockade was an act of economic warfare, to strangle the Confederacy’s trade with Europe. When Congress convened in July 1861, it quickly ratified the President’s move and authorized him to close ports of rebel-held areas.²⁷ Although the blockade’s military effectiveness is disputable, it produced a Supreme Court decision – one that still reverberates today – about the President’s power to make war.²⁸

Nineteenth-century blockades were governed by elaborate international rules for intercepting vessels coming in and out of ports. Under prevailing practice of the time, ships caught trying to break or “run” the blockade were subject to capture. When captured ships reached Union ports, federal “prize courts” awarded their value to the officers and crews who seized them. International law generally held that blockades were only valid in a state of war because they denied regular commercial freedoms to other nations.²⁹

The owners of four vessels captured and condemned as blockade runners between April and July 1861 sued. They alleged that the blockade was improper because no war had been declared at that time.³⁰ The Supreme Court ruled on these issues nearly two years into the war.

In a set of cases grouped together as The Prize Cases, the President had reason to worry. Chief Justice Taney was still seething over Lincoln’s rebuff in the Merryman case. Three of the nine justices at the time were Southerners, and a majority of the court had agreed with parts of Taney’s proslavery Dred Scott decision six years earlier. And, although the financial stakes were modest, the legal stakes were not. The Supreme Court could have ruled against the government on very broad grounds that called into question other exercises of war powers, such as the President’s practice of military detention. It could have held, for example, that a valid blockade, and perhaps by extension many other military actions, required a formal declaration of war by Congress – something Congress still had not done. Worse still, the Court might have called into question whether the national government could constitutionally wage war against seceding states at all.

By only a slim 5–4 margin, the Court upheld the early blockade actions, though all nine justices agreed that the President had at least some emergency military powers to defend the Union. Yet, the Court did not speak clearly on the source and boundaries of the President’s powers.

The case turned on whether it was the mere fact of war or an official declaration by Congress that triggered the wartime power to blockade enemy ports. The majority reasoned the former; what mattered was that at the time Lincoln instituted a blockade, he faced what was, in effect, a war. “This greatest of civil wars,” wrote Justice Robert Grier for the Court, “sprung forth suddenly from the parent brain, a Minerva in the full panoply of war. The President was bound to meet it in the shape it presented itself, without waiting for Congress to baptize it with a name; and no name given to it by him or them could change the fact.”³¹ Two opposing military forces were engaged, by any reasonable measure, in large-scale combat – in effect, war. In those circumstances, the majority reasoned, the President had authority to activate a blockade.

The four dissenting justices argued that a legal change from peace to war required a congressional declaration, or something like it. Until Congress, and only Congress, pulled that lever, the President lacked authority to institute a blockade, which international law reserved for wars. Embedded in the dissent’s reasoning are long-running arguments that certain war powers assigned in the Constitution to Congress may not be delegated to the Executive, emphasizing that a formal state of war had severe consequences for property rights like those of the litigating shipowners. And, again, the concern over which branch controlled making war was less about foreign policy risks and more about risks to core rights at home. “This great power over the business and property of the citizen,” wrote the dissenting four, “cannot be delegated or surrendered to the Executive. Congress alone can determine whether war exists or should be declared.”³²

Today, lawyers, scholars, and officials often cite The Prize Cases in arguing over the President’s power to use military force. The case makes for a useful precedent in part because Supreme Court decisions on the President’s and Congress’s powers to start wars are so rare. This remains a favorite case among advocates of unilateral presidential powers who read it to mean that Article II gives the President broad license to use military force to defend the United States. Usually, those advocates quote only a short fragment from the majority opinion: “If a war be made by invasion of a foreign nation, the President is not only authorized but bound to resist force by force.” Sometimes this quotation is clipped down even further and interpreted to mean that the President has unilateral constitutional power – even obligation – to defend militarily the United States and its citizens without awaiting congressional permission.³³ That reading would be consistent with the likely view of Framers that the President had some narrow, implied constitutional power to repel sudden invasions. Later chapters will show how modern executive branch lawyers stretch this reading to encompass many other scenarios, too.

Whatever the merits of broad presidential self-defense power, those modern interpretations may read too much into The Prize Cases. The Court left unclear whether the President’s authority to respond militarily to South’s rebellion was based entirely on his own constitutional power or statutes enacted by Congress. Lincoln’s blockade involved extraordinary factual circumstances, including that Congress was then out of session and later approved the blockade when it returned. What’s more, the Lincoln Administration’s arguments and the majority opinion emphasized that Congress had previously enacted two relevant laws discussed in Chapter 2: the Militia Act of the 1790s and the Insurrection Act of 1807. The Court stressed that those laws clothed the President in vast military powers to suppress rebellions with, respectively, state militias called into national service and the regular Army and Navy (all of which were directly or indirectly used to carry out the maritime operation). The Prize Cases therefore did not squarely test the President’s power to make war without any legislative backing whatsoever.

Indeed, neither the Court’s majority nor dissent reached their conclusions simply by looking to the President and Congress’s singular powers to make war. They did so by working through the interrelationships of many constitutional components, including: Congress’s powers to declare war, make rules for “captures” at sea, regulate militias, and regulate commerce; the purposes for which state militias may be used by the federal government; the President’s duty to faithfully execute the laws, his executive powers, and his role as commander in chief including differences between commanding regular armed forces and federalized militia forces; wartime property rights; the distinction between war, invasion, and insurrection; and the relationship between constitutional war powers and international law.

Despite this nuance, future executive branch lawyers would come to continually reach for this case as a clear-cut formula for the President’s defensive war powers. The endlessly repeated snippet that “the President is not only authorized but bound to resist force by force” feeds a line of thinking that simple claims of Congress’s sole power to make war are refutable with equally simple claims that the President may make war in response to attacks. However, The Prize Cases showed that the issue in 1861 was not straightforward, but deeply complex.

Sustaining War and Emancipation

The Prize Cases closed one constitutional chapter and opened another. If the President could act without Congress in the first shock of rebellion, what could the government as a whole do to win a modern war? On that question, the Court largely stood back.

The Civil War was a conflict of unprecedented scale, and waging it required mobilizing the entire Northern and Southern economies and societies. To carry on that kind of war, the U.S. Congress and the President took steps, including asserting control over the domestic economy, that would never have been considered constitutional in peacetime but were now justified constitutionally as necessary to win. Strategy and law reinforced each other.

The governmental powers needed to wage this war of endurance and efficiency made Lincoln not just commander-in-chief but manager-in-chief.³⁴ Meanwhile, both Congress and the President used their respective control over the means of war to influence its ends – in particular with respect to the war’s primary cause, slavery.

More than two million men served in the Union Army, with a peak strength of about one million – an order of magnitude larger than the U.S. force in the Mexican–American War.³⁵ Instead of old-fashioned muskets, industrial factories now churned out more accurate rifles. Trains and steamboats carried mass-produced ammunition and other war materiel to distant armies. Gettysburg alone killed more Americans in battle than the War of 1812 and the Mexican–American War combined. The war became one “in which the whole population and whole production” of the North and South, wrote one Confederate general, “are to be put on war footing, where every institution is to be made auxiliary to war.”³⁶ That was especially so as Union strategy shifted to destroying not just the Confederate army but its civilian infrastructure and supply lines.

The Union government ventured far beyond traditional policy to sustain this massive war effort. To raise enough troops, the Union turned in 1863 to a national draft for the first time in the republic’s history. So did the Confederacy.³⁷ As explained in Chapter 2, compulsory military service was not new to America, but compulsory federal military service was.³⁸ To finance the war, the national government also broke new ground, in the Legal Tender Act, by resorting to paper currency – “greenbacks,” because of the paper’s color – that was not backed by hard coin like gold or silver.³⁹ Chief Justice Taney, though no cases were before the Court, privately drafted two opinions in which he concluded that both federal conscription and paper currency were unconstitutional.⁴⁰

The Supreme Court never addressed the national draft during the Civil War. The Pennsylvania Supreme Court did, however, briefly striking it down before reversing itself when a new justice shifted the majority. The U.S. Supreme Court performed a similar about-face on the Legal Tender Act in a pair of cases decided well after the war ended. First, it struck down that act as beyond Congress’s limited powers to interfere with private contracts, and then, it overturned itself a year later to uphold the legislation as necessary to support the war (also after some membership turnover on the Court).⁴¹

These and other Civil War moves established a momentous legal formula: New types of warfare require new means to wage them victoriously, and constitutional powers – especially Congress’s – flex to include those means. This legal move resembled, and sometimes drew upon, a parallel formula in international law, as the Union interpreted the laws of war to incorporate the principle of military necessity – a principle that both licensed actions needed to subdue the enemy and limited military means to those needs.⁴² Whatever rhetorical variations Civil War leaders may have used, many constitutional questions boiled down to a logic of military necessity; that is, rather than an exception to the rules, military necessity was a built-in part of constitutional law. All three branches of government embraced this understanding to a large extent, but none of them offered a limiting principle in a protracted war for national survival.

The most dramatic application of this formula concerned the war’s central issue: slavery. At the outset, Lincoln had made clear that this was not a war to abolish slavery. Whatever his moral abhorrence of the practice, the President’s early strategic priority was keeping slaveholding border states on the Union side, and he feared that any suggestion that dismantling slavery was a war aim would push those states over the secessionist edge.⁴³ Abolitionists in Congress, on the other hand, had few qualms about using the war to advance their agenda. Throughout the war, debates about slavery within and between the elected branches were filled with arguments about their respective war powers and whether antislavery policies could be justified as war measures.

For their part, congressional abolitionists passed a pair of Confiscation Acts in 1861 and 1862, which authorized Union forces to seize certain types of rebel property and to liberate many enslaved persons – first those who escaped and whose labor had been used on behalf of the Confederacy’s war, and later those of disloyal owners. Those statutes were constitutionally contentious for several reasons. Even the most ardent abolitionists recognized that the national government lacked the power to restrict slavery within the states during peacetime. But could emancipation be justified constitutionally as a war measure, pursuant to a belligerent’s right to seize enemy property? And if the national government did have an implied war power to free rebels’ slaves, which branch (or branches) held that power?

On the first question, Republican legislators argued that property confiscation was a military measure essential to winning the war. Some lawmakers justified the Confiscation Acts by appealing to international law, claiming that seizing enemies’ property used in waging war was a customary right of belligerents. It was lawful, for example, to seize or burn Southern crops being used to feed Confederate troops. “Therefore,” declared Radical Republican Senator Charles Sumner after quoting the most well-regarded international law publicists of the time, “according to the Rights of War, slaves, if regarded as property, may be declared free; or if regarded as men, they may be declared free.” Sumner later declared in Congress: “There is not one of the rights of war which Congress may not exercise; there is not a weapon in its terrible arsenal that Congress may not grasp.” Although Congress’s constitutional powers might be strictly limited in peacetime, he was arguing, in wartime they implicitly included practices recognized by international law as militarily expedient.⁴⁴ Earlier chapters discussed times when the President made this legal move to inflate commander-in-chief authority. Here was Congress making it – and thereby driving a U.S. military strategy of liberating enslaved people.

But if the national government had the wartime power to free enemy slaves, as a necessary incident to war and sanctioned by international law, which branch held the power? Was it Congress, to which Article I assigns powers to enact legislation “necessary and proper” to carrying out the Constitution’s other powers, as Sumner suggested? Was it the President, whom Article II designates the Commander in Chief? Was it both?

To some supporters of emancipation, this second-order question mattered little. But it raised the vexing and recurring issue of how war powers interlink, with some Republicans in Congress questioning whether Congress was overstepping by tying the chief commander’s hands.⁴⁵ After all, so-called Radical Republicans behind these legislative acts were deliberately pushing a more cautious Lincoln, the Commander in Chief, to conduct aspects of the war more aggressively than he initially thought strategically wise. Given the centrality of slavery to the war’s causes and conduct, the Confiscation Acts stand as an extraordinary congressional management of how it was waged.

Had the President and Congress stayed divided on slavery, clashes over confiscation legislation might have forced a showdown about presidential control of the war’s conduct. But as the war progressed, Lincoln’s views shifted from seeing emancipation as a strategic liability to a necessity. Perhaps unsurprisingly, his views on the constitutionality of emancipation shifted accordingly, as did his sense of its international legality. By 1862, building on Congress’s Confiscation Acts, Lincoln resolved to issue a sweeping emancipation order of his own, on military necessity grounds – seeking to gut the Confederacy’s military and economic reliance on slave labor. As a policy matter, this move unmistakably transformed the war for the Union into a war against slavery. Its legal footing, by contrast, remains unclear.

Lincoln’s celebrated January 1, 1863, Emancipation Proclamation – which declared free “thenceforward, and forever” all those enslaved in areas still in active rebellion – is an astounding edict, not just because of its impact but because he issued it “by virtue of the power vested as Commander-in-Chief, of the Army and Navy of the United States in time of actual armed rebellion.” To many of its supporters, this decree was a noble invocation of war powers to enlarge liberty. To many of its opponents, it epitomized the gravest dangers of turning war powers into instruments of tyranny.

Lincoln never explained the Emancipation Proclamation’s legal rationale in any detail – to the extent there was any single rationale – and scholars have split over how to infer it. Some scholars emphasize legislative war powers as the root source of authority, pointing out that Congress had already mandated steps toward emancipation with the Confiscation Acts. In his September 1862 “Preliminary Emancipation Proclamation” – which warned that if Confederate states did not return to the Union by January 1, all enslaved people in states still in rebellion would be declared free – Lincoln expressly cited that legislation. That warning and then Lincoln’s January 1863 decree went further than any statutes, however, by covering all slaves – not just those enslaved by individuals supporting insurrection – in rebellious Southern areas. Lincoln also had on earlier occasions expressed doubts about Congress’s power to free slaves by law (though he had also expressed doubts about the President’s power to do so).⁴⁶ When Lincoln solicited comments on his draft days before issuing the final Emancipation Proclamation, his Attorney General urged him to emphasize as its basis the President’s power to take war measures, “not a law by the legislature.”⁴⁷ The final Proclamation makes no mention of statutory authorization at all, perhaps suggesting that Lincoln’s actions rested on presidential war powers alone – but that point remains ambiguous, too.

If the President, Congress, or both may wield a constitutional power to do whatever is militarily necessary in wartime, does this power exist without limit? Those Framers who worried that war would expand national government powers at the expense of liberty would have been astonished by the arguments and actions of Lincoln and the Radical Republicans. But this disjuncture is a reflection less of disagreement over constitutional principle and more of the revolution in warfare itself that took place between the Founding era and the Civil War. In the War of 1812, when their national charter was just adolescent, the Founding generation confronted their own dilemmas and constitutional controversies over how to pay and recruit soldiers for a protracted war. These were familiar administrative issues from the Revolutionary War, though. The Constitution also specifically addressed those matters in Article I, even if the text left much room for debate. The Civil War was a new type of conflict, not just because it was internal but because its scale required mobilizing the entire national economy and society. Such warfare would have been pure fantasy at the Founding. As war grew in many dimensions – including the stakes – so did the government’s power to wage it.

One might expect courts to have checked such an open-ended maxim, but on the whole, the Civil War judiciary deferred heavily to the elected branches’ judgement of necessity – especially when Congress acted. Chief Justice Taney’s rejection of Lincoln’s necessity argument in the Merryman case, which involved unilateral presidential action, is a notable exception. As mentioned earlier in this section, the Supreme Court initially struck down legislation to fund the war with paper-only currency, but it quickly reversed itself, declaring: “It is not to be denied that acts may be adapted to the exercise of lawful power, and appropriate to it, in seasons of exigency, which would be inappropriate at other times.” Why was this legislation, likely unconstitutional in peacetime, constitutional during the war? “Suffice it to say,” wrote the Court, that the Civil War threatened destruction of the republic altogether – and waging it “demanded the equipment and support of large armies and navies, and the employment of money to an extent beyond the capacity of all ordinary sources of supply.” Congress must be afforded leeway, in other words, to match wartime powers to wartime exigencies.⁴⁸ Indeed, the Supreme Court even suggested that courts might not have any role in questioning Congress’s assessment of wartime necessity – and therefore the scope of Congress’s own power.

After four years of war – through mass conscription, fiscal revolution, emancipation, and other measures – the legal logic of necessity had carried the Union government farther than in any previous conflict and farther than most Framers could have imagined. Upon victory, the nation would have to grapple with fraught constitutional adjustments to restore peace.

War Ends and Ending Wars

In April 1865, Confederate General Robert E. Lee formally surrendered at Appomattox, Virginia. Only a week later, Lincoln would be assassinated and Vice President Andrew Johnson would assume the presidency. Lee’s capitulation was the decisive military blow of the four-year war, but combat operations and guerrilla resistance continued elsewhere in the South and secessionist states still had to be readmitted.

Five years on, the Supreme Court in Stewart v. Kahn upheld a wartime act that suspended statutes of limitations from tolling while the conflict limited access to courts. In that 1870 decision, the Court remarked that “the [war] power is not limited to victories in the field and the dispersion of enemy forces. It carries with it inherently the power to guard against the immediate renewal of the conflict, and to remedy the evils which have arisen from its rise and progress.”⁴⁹ The Court appeared to be saying that the reach of Congress’s war powers is not tied simply to the needs of subduing the enemy militarily but extends to measures necessary to a lasting peace and to addressing problems unleashed by the war. For years after major combat ended, Congress indeed leaned on its constitutional war powers to sustain Reconstruction – justifying a substantial military presence in the South, governance through military districts, and measures to protect freed people and suppress political violence. In doing so, lawmakers blurred the line between wartime and peacetime authority, treating Reconstruction as an extension of war even as it sought to restore the Confederate states to the Union.⁵⁰

Such a spacious view of Congress’s war powers thus raised another question of enormous consequence that was not squarely asked and answered in the Civil War. It is a question that, as later chapters will show, bedevils constitutional law to this day: If expanded wartime powers are tied not just to achieving battlefield victory but to securing durable peace and return to normalcy, when do those extraordinary powers retract?

A Union victory could never have been marked with a customary peace treaty. After all, the Union strenuously denied that the Confederacy was a sovereign state with which it would negotiate on those terms. So when did the war end? When major combat ended? When all remaining pockets of rebellion were quashed? When the President declared the war over? When Congress did? When the last Confederate state was readmitted to the Union?⁵¹

One reason the answer mattered – and would have mattered even more in an interstate war – had to do with external relations. The conclusion of nineteenth-century conflicts did not merely halt the fighting. Under prevailing international law, it also marked the withdrawal of belligerent privileges, like the right to blockade enemy ports, and the resumption of peacetime diplomatic and commercial dealings with neutral states. In this case, that meant reopening Southern trade with Europe.

Another reason had to do with the endurance of wartime governance at home. The Union government – both Congress and the President – had been asserting constitutional war powers as the basis for exceptional measures like emancipation and wide-ranging economic mobilization. If the start of war triggers or expands some war powers, then its endpoint would presumably halt or narrow them.⁵² This issue is something of a mirror image to the questions presented in The Prize Cases. While that case asked what conditions activate wartime constitutional powers at the front end of conflict, the winding down of combat raised the question of when those powers expire at the back end.

The condition of violent armed conflict can be objectively assessed – the Supreme Court did that in The Prize Cases – and the condition of peace can likewise be observed. But “victory” is a political concept. Ambitious ends might beget persisting constitutional means to achieve them, even after the fighting subsides.

Important follow-on questions therefore abound, too, including whether the President or Congress controls the termination of war and war powers – the Republican Congress wanted to extend them well beyond the point that President Johnson did – and whether courts have a role in checking those decisions. The more the substantive scope of war powers gets stretched – and in the Civil War, they were stretched very far – the more important the durational limits become.

In the end, none of the weightiest questions about ending wars and lapsing war powers were answered clearly. They were largely mooted when Congress acquired new sources of constitutional power following the Union’s battlefield triumph: the Reconstruction Amendments. Late into the 1860s, some members of Congress continued to assert war powers as the constitutional basis for Union Army occupation of the South and measures to protect freed people. But among other bases, the Thirteenth Amendment (ratified in 1865) banning slavery and the Fourteenth Amendment (ratified in 1868) granting all persons equal protection under state laws conferred on Congress vast new “power to enforce” the new constitutional rights “by appropriate legislation.”

Even if Congress’s general war powers ceased or tapered off, those new amendments largely picked up the slack, leaving little urgency to determine when legislative powers had formally retracted. As Chapter 6 will show, World War I would raise that problem far more sharply. For now, the endurance of emergency military governance – where and how long it could outlive the fighting – would soon come before the courts.

Martial Law and Military Commissions

Lambdin Milligan was set to be hanged on May 19, 1865, six weeks after Lee’s surrender and a month after Confederate sympathizers assassinated Lincoln. The previous year, Union troops had detained the former Indiana politician in the middle of the night, based on suspicions that Milligan led the military wing of a hostile secret society. Charged with plotting an armed uprising against the Union, he and a band of coconspirators were carried off to Indianapolis, where they were convicted and sentenced to death not in a civilian court but in a military commission: a tribunal of officers established to try individuals for unlawful conduct associated with the war. The Union leadership viewed military commissions as a means to delivering swift justice, bypassing certain constitutional protections such as a jury of peers.

Milligan’s conviction and sentence were set aside the next year by the Supreme Court for its failure to afford him those rights. “No graver question was ever considered by this court,” claimed the majority opinion in that case, “nor one which more nearly concerns the rights of the whole.”⁵³ That was a huge overstatement, but the case involved a momentous clash of competing ideas about the Constitution in wartime.

Neither “military commissions” nor “martial law” are provided for in the Constitution, but both have deep histories in American practice. Military commissions, whether called that name or something else, had been used, among other times, in the Revolutionary War, the War of 1812, and more extensively to try civilians who impeded military operations during the Mexican–American War.⁵⁴ Martial law is a broader concept, usually meaning the entire displacement of civilian government and processes of justice by military rule, often within a specific area. It, too, had deep American roots. Near the end of the War of 1812, for example, then-general Andrew Jackson imposed martial law in New Orleans, and Chapter 3 discussed its use in the Mexican–American War.⁵⁵

During the Civil War, Union generals imposed martial law in many locales, sometimes with and sometimes without Lincoln’s approval – and that practice included the use of military commissions to prosecute thousands of rebel guerrillas and agitators. The most common decrees of martial law were in active combat zones, areas of the Confederacy captured by Union forces, and regions in which guerrillas and saboteurs threatened Union government or operations. The first two were less constitutionally controversial than the third, which was at issue in Milligan’s case. Union military leaders in Indiana (a free state on the border of former slave states) feared that shadowy anti-war groups like Milligan’s could spark local uprisings, so they resorted to military crackdowns. Lincoln had earlier proclaimed under his own constitutional authority that military commissions would be used as a necessary war measure against rebels and their abettors. Now, in defending its actions against Milligan, the government argued that the President’s power to command military forces included the power to capture and punish through military commissions those who armed the enemy and abetted insurrection.⁵⁶

Writing for a five-member Supreme Court majority, Justice David Davis held Milligan’s military conviction and sentence unconstitutional. Passionate newspaper editorials divided along partisan lines, portraying the decision as either vindicating war-critics’ allegations of dictatorship or judicially undermining government efforts to secure Union victory and carry out Reconstruction.⁵⁷ The Court reasoned that Milligan was entitled under the Bill of Rights to a trial by jury and other civilian-court rights because he was not a member of an enemy armed force and Indiana’s civilian courts were functioning. Only “[i]f, in foreign invasion or civil war, the courts are actually closed, and it is impossible to administer criminal justice according to law,” the majority reasoned, “then, on the theatre of active military operations, where war really prevails,” the military “is allowed to govern by martial rule until the laws can have their free course.”⁵⁸ Going further, and drawing conclusions beyond the facts of the case, it asserted that not even Congress had the power to legislate military commissions in situations like Indiana’s.

Chief Justice Salmon Chase, in a concurrence joined by three other justices, thought the Court was wrong to reach that last issue. Also drawing conclusions beyond the facts at hand, the concurring justices wrote separately to argue that Congress could have authorized military commissions like the one that convicted and sentenced Milligan, but that Congress had declined to do so.⁵⁹

It was formally a case about wartime measures, but its implications ran deep into the defining policy battles that followed. By undercutting commissions, Milligan hindered emancipation’s enforcement in the occupied South, where civilian courts failed to protect Unionists and the formerly enslaved. As John Witt explains in Lincoln’s Code, some members of the Administration’s legal team “were not really arguing about Lambdin Milligan at all. Their arguments were calculated not to sustain his conviction but to defend the authority of military commissions and martial law during postwar Reconstruction.”⁶⁰ Here was constitutional law constraining war- or postwar strategy – to include Congress’s strategy at the time of sustaining military governance of the South – depending on how one divides those periods.

Beyond its practical impact, however, the Milligan case stands as an outlier in Supreme Court decision-making. Courts usually defer to the elected branches on the scope of war powers when Congress and the President act in unison.⁶¹ One explanation for this anomaly is timing. A year had passed since Lee’s surrender at Appomattox when the Supreme Court ruled in Milligan, and only days earlier, President Andrew Johnson had issued a proclamation declaring the Confederate insurrection to be over (though Congress would push back, including by continuing to justify Reconstruction legislation and military rule of Southern states under its war powers).⁶²

Indeed, there is strong evidence that the Milligan case would likely have come out differently at the height of the war. Justice Davis, writing for the majority, practically said so in stating, “During the late wicked Rebellion, the temper of the times did not allow that calmness in deliberation and discussion so necessary to a correct conclusion of a purely judicial question.”⁶³ When a similar case had reached the Court two years earlier – at that time, former Ohio Congressman Clement Vallandigham was sentenced by a Union military commission for anti-war speeches – the Supreme Court refused to touch the case, ruling that the Court lacked jurisdiction to review the matter.⁶⁴ Two years later, with smoke from the war finally clearing, the Milligan Court took on the issues.

The most notable aspect of the Milligan decision is not its specific holding but the clashing theories of war powers voiced by different justices. The majority five began their analysis by emphasizing that the Founders fixed rigidly strict limits on government authority, knowing that wartime emergency might create political pressure to grab excessive power. “Those great and good men foresaw that troublous times would arise when rulers and people would become restive under restraint, and seek by sharp and decisive measures to accomplish ends deemed just and proper, and that the principles of constitutional liberty would be in peril unless established by irrepealable law.”⁶⁵ This view sees some constitutional rigidity as a deliberate design feature for war.

If the majority emphasized the danger of too much constitutional flexibility, the four concurring justices emphasized the danger of too little. They stressed that because Congress has the power to create armed forces and declare war, it must also implicitly have “the power to provide by law for carrying on war,” including power extending to “all legislation essential to the prosecution of war with vigor and success except such as interferes with the command of the forces and the conduct of command.”⁶⁶ This view sees the Constitution as engineered for wartime elasticity.

The debate over whether the Constitution is devised for war-flexibility or war-rigidity recurs throughout American history. The Milligan majority’s constitutional diagram looks like the model sketched by Daniel Webster in arguing against a national draft in the War of 1812: “The People have granted all the means which are ordinary and usual, and which are consistent with the liberties and security of the People themselves, and they have granted no others. To talk about the unlimited power of the government over the means to execute its authority is to hold a language which is true only in regard to despotisms.”⁶⁷ The Milligan concurrence, by contrast, resembles Hamilton’s layout in Federalist Papers, envisioning that war powers “ought to exist without limitation, because it is impossible to foresee or define the extent and variety of national exigencies.”⁶⁸

As later chapters will show, within that enduring debate over how the Constitution is engineered for war, the Milligan case stands out as an occasional exception – a moment when a Supreme Court majority chose rigidity even after the elected branches had tried to bend constitutional lines in wartime. It was an especially striking outlier in a conflict otherwise defined by the Constitution’s extraordinary elasticity.

Conclusion

The Civil War tested the Founding charter like never before: It was truly a war for the republic’s very preservation, waged at a scale that stressed the nation nearly to the breaking point. Nearly – but the constitutional system survived.

Across so many specific controversies, three general tensions stand out. First, the Civil War turned upside-down many constitutional Founders’ assumptions about concentrated military power as a threat to liberty and republican government. Now, the war aims – to preserve the Union from secession and, later, to abolish slavery – framed centralized federal authority, including military power, as an essential counterforce to the denial of liberty by states.

Second, the national government claimed legal powers that were virtually limitless to preserve a constitutional system premised on limited powers. Constitutional war powers were stretched not only by the gravity of the stakes but by the unparalleled scale of modern warfare. With momentary exceptions, all three branches of government endorsed almost infinitely elastic war powers to save a system opposed to absolute rule.

Third, the Civil War produced extensive legal reasoning about the Constitution’s special meaning in war, even though ultimately it took brute military victory to establish lasting constitutional precedents. This savage conflict of national survival is marked by abundant and painstaking constitutional arguments about war powers in executive branch documents, congressional speeches, and Supreme Court opinions, along with political pamphlets, newspaper editorials, and officials’ private correspondence. Legal debate flourished amid the turmoil – sometimes bolstering, sometimes curbing strategy.

One might be tempted to dismiss much of that legalism as just a thin legal veneer for raw force. After all, it was the Union’s battlefield triumph more than any authoritative legal rulings that in the end validated sweeping assertions of presidential and congressional war powers. Yet constitutional arguments were integral to wartime politics and to justifying publicly the wartime measures that were key to military success. Law became its own theater of war. To a remarkable degree and throughout the entire conflict, defenders and detractors of the Union campaign to preserve the nation by force – and of specific strategic choices in that campaign – appealed to the Founding charter’s special design features for war.

Later chapters show that the struggle over constitutional powers for wars – or potential wars – of survival would recur. They also reveal that some of the radical constitutional formulas born in this extraordinary conflict, fought for the nation’s very existence, set precedents that would guide more ordinary wars. In the immediate decades ahead, however, the United States would enter a new era in which the practice of war was no longer about survival – that was secure, for now – but about the role the nation would play in the world.