It is 1945 and Generaloberst Lothar Rendulic has received an order from the Nazi High Command that no commander would welcome: to instantly retreat in the face of a projected offensive by a superior armed force through the territory of a former cobelligerent. At the time, General Rendulic was the commander the 20th Mountain Army of the Wehrmacht, Nazi Germany's unified armed forces, as well as of all German troops stationed in Finland and Norway. Finland had just entered into a separate peace with Russia following the War of Continuation (1941–1944) and demanded the immediate withdrawal of German troops. Rendulic was thus left to fight his way out of Finland and into Norway, which had been occupied by Germany since 1940 with the help of the collaborationist Quisling regime.

Rendulic faced a daunting task. Operating under the assumption that the Soviet Army was in quick pursuit, Rendulic launched Operation Nordlicht—“Northern Light”—a scorched earth retreat through the northern Norwegian province of Finnmark, destroying entire towns in an effort to keep the Soviets at bay while denying them aid and comfort. The operation left thousands of civilians internally and externally displaced and much of the civilian infrastructure devastated.Footnote ¹ In the words of the military tribunal that would later adjudge the legality of this course of conduct, “[t]he destruction was as complete as an efficient army could do it.”Footnote ² In the end, Russian troops did not give chase, so this strategy was for naught. Exiled Norwegian troops billeted in Sweden and Great Britain eventually liberated the area, fighting alongside Soviet troops and Norwegian civilian defense units manned by those who survived the arctic winter by hiding in forests and caves. After an abortive last stand in Norway, the German army capitulated in May 1945.

Rendulic was later prosecuted by the United States under Control Council Law No. 10 in the so-called Hostages Trial, which involved allegations of war crimes in a number of occupied and allied territories. Among other charges, Rendulic was indicted for plunder and wanton destruction in the absence of military necessity.Footnote ³ The U.S. military commission, however, acquitted him on these charges. In so doing, it formulated what has become known as the Rendulic Rule—essentially that a commander's evaluation of military necessity must be judged not in retrospect, on the basis of the conditions as they later proved to be, but rather in light of the circumstances as they appeared to the commander at the time, bearing in mind all the uncertainties occasioned by the proverbial fog of war.Footnote ⁴ The commission thus explained:

Rendulic and his codefendants were convicted, however, of other charges—including the murder, mistreatment, forced labor, and deportation to slave labor of members of the civilian populations of Greece, Yugoslavia, and Albania; the denial of quarter of Allied prisoners of war; and participation in reprisal killings and the taking of protected persons hostage.Footnote ⁶ In this portion of the verdict, the military commission rejected the defense of military necessity, reasoning that it “permits the destruction of life of armed enemies and other persons whose destruction is incidentally unavoidable by the armed conflicts of the war . . . but it does not permit the killing of innocent inhabitants for purposes of revenge or the satisfaction of a lust to kill.”Footnote ⁷ Although sentenced to twenty years’ imprisonment,Footnote ⁸ in the end, Rendulic served only ten years—the beneficiary of a dramatic Cold War realignment, which positioned Germany as a crucial bulwark against the Soviet Union's fearsome western expansion.

Given this history, it is thus fitting that the book that will undoubtedly become a one-stop source for the concept of military necessity would be written by Sigrid Redse Johansen, judge advocate general for the Norwegian Armed Forces. In The Military Commander's Necessity: The Law of Armed Conflict and Its Limits, Johansen identifies the functions military necessity serves within international humanitarian law (IHL), its elements and substantive content, and the “legal limits of the commander's assessment of military necessity” during the conduct of hostilities (p. 399).Footnote ⁹ In so doing, she surveys the core IHL treaties and their travaux préparatoires, state practice and expressions of (il)legality surrounding a number of historical conflict events, case law emerging from the war crimes tribunals (particularly the International Criminal Tribunal for the former Yugoslavia (ICTY)), military manuals,Footnote ¹⁰ and lingering doctrinal debates. In many respects, the book's title undersells Johansen's contribution, which amounts to a comprehensive treatment of much of the law of war through the lens of military necessity—one half of the grand, but delicate, balance between military exigencies and humanitarian concerns that undergirds all of IHL.Footnote ¹¹

The Lieber Code, which governed the Union Army during the U.S. Civil War, contained a definition at Article 14 that retains considerable currency today: “military necessity, as understood by modern civilized nations, consists in the necessity of those measures which are indispensable for securing the ends of the war, and which are lawful according to the modern law and usages of war.”Footnote ¹² The principle appears in the modern corpus of IHL in two incarnations: as an affirmative justification to deploy kinetic force—within constraints—in pursuit of a war aim and as an express exception to various prohibitive rules. Given this enigmatic nature, Michael Schmitt has noted that the principle “has been proffered both to justify horrendous abuses during armed conflicts and to impose impractical and dangerous restrictions on those who fight.”Footnote ¹³

Johansen cabins this principle by establishing that it neither imposes an additional restraint upon conduct that is otherwise in conformity with IHL (in other words, a commander need not justify the military necessity of a strike against a military objective), an argument human rights advocates will raise; nor does it provide a freestanding defense to any particular course of action (in other words, a claim of military necessity does not override the law), a critical point often lost in some national security rhetoric.Footnote ¹⁴ Rather, she posits that military necessity has now been completely absorbed by particular IHL rules.Footnote ¹⁵ The architects of these rules have endeavored to reconcile the often-competing imperatives of military necessity and humanitarian concern and, in so doing, police the line between conduct that is presumptively valid and wanton acts that are always prohibited because they lack military necessity (p. 1). Crucial to this full-scale IHL incorporation was the adoption of Additional Protocol I to the 1949 Geneva Conventions.Footnote ¹⁶ Indeed, debates over military necessity explain the persistent objections of the United States (and other influential states) to certain of the Protocol's provisions, in part on the ground that they fail to adequately take military necessity into account.Footnote ¹⁷

Military necessity has a long pedigree in the law of armed conflict, and each chapter of Johansen's book begins with a backward glance to the historical circumstances that led to the articulation of the rule, its intersection with military necessity, and real-world instances of its application (and abuse). Johansen demonstrates that military necessity undergirds many of the core concepts of IHL—including the principles of distinction and proportionality, the prohibition on superfluous injury and unnecessary suffering, the special protections accorded to certain classes of person and property, and the imperative to undertake precautions in attack, but only if feasible—all of which receive careful treatment in dedicated chapters in the book. Military necessity is also at the heart of contemporary debates over the lawfulness of attacks on military objectives encumbered by human shields,Footnote ¹⁸ on civilians who directly participate in hostilities,Footnote ¹⁹ on war-sustaining activities,Footnote ²⁰ and with lethal autonomous weaponsFootnote ²¹—vexing topics that get less attention than they deserve in the text.

It is axiomatic that every balancing test requires the relevant—and potentially incommensurate—factors to be weighed against each other. In IHL, it is the military commander, in headquarters or in the field, who takes the first, but not necessarily the final, crack at this calculation. The book reveals areas where the law imposes heavy burdens on military commanders in exercising this discretion given the imprecise and open-textured formulation of applicable rules.Footnote ²² The IHL scholarship stands to benefit from such efforts to transcend the theoretical underpinnings and abstract formulations of the rules and instead think more concretely about how real human beings must apply them in the real world or suffer the consequences. The book is at its best when it deconstructs actual military practice in historical operations, such as the North Atlantic Treaty Organization's (NATO) Unified Protector Operation in Libya following the issuance of UN Security Council Resolution 1973 of 2011,Footnote ²³ or the 1991 Operation Desert Storm attack on the Al-Firdos bunker, which U.S. intelligence thought was being used as a C3 hub for Saddam Hussein's forces but was in fact sheltering hundreds of civilians.Footnote ²⁴

As the Rendulic Rule makes clear, commanders enjoy a measure of discretion and operational latitude in the conduct of hostilities.Footnote ²⁵ Commanders—at least the “reasonable” ones—are assumed to be acting in good faith while operating in the face of what might be rapidly evolving circumstances and profound uncertainty, although Johansen concedes that “the distinction between excusable and culpable lack of information may be a fine line in practice” (p. 78). Johansen borrows the term “margin of appreciation” from the jurisprudence of the European Court of Human Rights (ECtHR) to capture this measure of discretion.Footnote ²⁶ According to this construct, the ECtHR grants member states some flexibility in how they implement their obligations under the European Convention for the Protection of Human Rights and Fundamental Freedoms given their different legal and cultural traditions and the lack of a European consensus around some issues. The margin of appreciation is not static; rather, it expands and contracts depending on various factors. States are accorded narrow margins of appreciation when, for example, the right implicated involves an important facet of an individual's identity, when a course of conduct suggests discrimination on invidious grounds, when the state seeks to restrict absolute rights, or when the authority or independence of the judiciary is under threat. On the other hand, where there is no European consensus on an issue, in situations of national security or public emergency, or when the state is ostensibly protecting morals or implementing social and economic policies, states enjoy a wider margin of appreciation.Footnote ²⁷

Likewise, the commander's discretion under IHL is not unlimited or uniform. Johansen lingers on areas in the law that appear to leave considerable room for the commander's discretion in determining military necessity, such as in gauging the status of prospective targets and applying proportionality in the face of potential “collateral damage,” particularly in connection with so-called dual use objects. In some circumstances, the “discretionary choices incumbent upon the commander . . . appear almost infinite” (p. 406). Many of the areas where the limitations on the commander's assessment are “vague, ambiguous and wide” are the result of negotiating states kicking the can down the road by way of compromise (p. 400). The text makes its most innovative contributions when it focuses how to evaluate the commander's exercise of this discretion, given the inherent subjectivity of certain assessments and the evolving dialectic between the exigencies of the battlefield and humanitarian concerns.

There are a number of ways that the commander's discretionary judgment may itself be judged ex post. These include internal after actions reviews, military disciplinary proceedings, a regime of state responsibility (given that the commander's actions can be attributed to the state and generate a duty to pay reparations),Footnote ²⁸ political oversight,Footnote ²⁹ and war crimes trials.Footnote ³⁰ Johansen focuses on the latter, noting that the courts have made significant contributions to IHL over the years, particularly after World War II,Footnote ³¹ continuing in sporadic cases during the Cold War, and then in a steady flow since the international criminal law renaissance in the mid-1990s.Footnote ³² Like many military lawyers, Johansen raises concerns about international judges substituting their judgment for that of the commander. Indeed, there are a number of opinions in which international judges have gotten IHL painfully wrong.Footnote ³³ Although one might share her skepticism of the ability of some international judges—who may arrive to the bench from academia, diplomacy, or some field other than the battlefield—the reality is that there are few other avenues for authoritative interpretation and enforcement, particularly as the international community moves—albeit haltingly—toward the judicialization of international law.Footnote ³⁴ This underlines the importance of states choosing tribunal principals with backgrounds in IHL and participating actively in litigation through party and amicus briefsFootnote ³⁵ (rather than withdrawing or reacting in ways that undermine the rule of law).Footnote ³⁶

A few observations on the scope of Johansen's text. The book focuses only on the law governing international armed conflicts—a significant limitation given that today's conflicts are primarily noninternational, or mixed, in nature.Footnote ³⁷ To be sure, the exercise of conflict classification has become less pressing given the unified corpus of rules applying across the conflict classification spectrum. That said, it remains important to questions of lawful combatancy, the status of fighters, and the entitlement to prisoner of war status—areas where military necessity has been invoked to justify abuses. Although the book is very much written from the perspective of a state actor commanding national armed forces against a sovereign foe—no surprise given Johansen's background—more thinking needs to be done on how the concept of military necessity might operate differently when the commander in question is engaged in a counter-insurgency operation, trying to win hearts and minds, or is leading nonstate actors, and thus engaged in combat without the privilege of doing so.Footnote ³⁸ This is an area that has been woefully undertheorized and could benefit from greater scholarly attention, especially from authors with field experience. The book's failure to grapple with how this principle applies in noninternational armed conflicts threatens to render it more of a historical hornbook than a text with pressing contemporary application.

Other areas that the book does not traverse include the law of belligerent occupation, coalition warfare, and the interface between international human rights law and IHL.Footnote ³⁹ While the absence of the former two topics might be forgiven, it is unfortunate that the author does not delve more deeply into the inquiry of when human rights law might tip the juridical scale away from military necessity and toward humanitarian concerns. This is most apparent in the “capture versus kill” debate inspired by Jean Pictet's use of force continuum and taken up by the International Committee of the Red Cross (ICRC) in its Interpretive Guidance on Direct Participation in Hostilities: “[i]t would defy basic notions of humanity to kill an adversary or to refrain from giving him or her an opportunity to surrender where there manifestly is no necessity for the use of lethal force.”Footnote ⁴⁰ To be sure, there is little positive law on this point,Footnote ⁴¹ and there no question that this revisionist account remains a minority opinion,Footnote ⁴² but there is a lively and important debate in policymaking circlesFootnote ⁴³ and emergent scholarship that is only hinted to in the text.Footnote ⁴⁴ Indeed, one wonders whether the military commission that acquitted Rendulic after World War II would reach the same result today, even applying the rule that bears his name.

Johansen's thick description of IHL, which yields a density of prose, is not appropriate for the casual reader, but will be appreciated by IHL aficionados, particularly those representing a sovereign. While the text is adept at diagnosing areas of the law where reasonable minds might reach divergent opinions, the reader is left with less guidance on how these dilemmas should be resolved, either by commanders ex ante or those who would judge them ex post.Footnote ⁴⁵ Nonetheless, Johansen's contribution is an important one that adds to the repudiation of the discredited, but stubbornly irrepressible, concept of kriegsraison geht vor kriegsmanier—the assertion that “necessity knows no law.” Most intimately associated with the Prussian military strategist Carl von Clausewitz, who at one point championed the concept of total war,Footnote ⁴⁶ Kriegsraison reached its apotheosis in World War I Germany. It continues, however, to be resurrected in some circles when it is forgotten that “savagery is actually counterproductive.”Footnote ⁴⁷