On the night of December 29, 1837, a British-led Canadian team crossed the Niagara River to New York and raided the U.S. steamer Caroline, setting it aflame and sending it over the Falls. At least one American was killed. Britain claimed that it had acted to prevent the Caroline being used to supply William Lyon Mackenzie's armed insurgency against colonial rule in Upper Canada. The United States claimed that Britain had used force illegally on American sovereign territory. Aided by inflammatory media coverage, the recriminations escalated. For several years, war loomed.

It was only after governmental and diplomatic changes in both states and the acquittal of an alleged member of the raiding party by a New York jury that each began to step back from the brink. In mid-1842, American Secretary of State Daniel Webster wrote to his new British interlocutor, Lord Ashburton (Alexander Baring). Webster attached a letter he had initially sent over a year earlier, arguing that the raid's legality would hinge on whether Britain could show both a “necessity of self-defence, instant, overwhelming, and leaving no choice of means and no moment for deliberation,” and that it eschewed any measures that were “unreasonable or excessive” in relation to that imperative (p. 118). Ashburton endorsed this formula immediately and without reservation (p.119). The parties agreed to disagree on whether Britain had in fact satisfied this test (pp. 120–21).

The Caroline affair has since become one of the most frequently cited incidents in the modern history of the jus ad bellum. Indeed, the précis above is likely known more or less by rote to any student, scholar, or practitioner of the law on the use of force. And yet, the raid, the diplomatic exchange, and its legal legacy have long been only hazily understood, and often fundamentally misrepresented, including in leading treatises, state pronouncements, and judicial decisions.

In a gripping text, University of Ottawa professor Craig Forcese seeks to rectify that failing, examining anew and in detail the events of 1837–1842, charting the invocation of the affair since, and grappling normatively with the implications of Webster's legal formula in the contemporary global security context. The book makes an indispensable contribution to the jus ad bellum literature. It blends history, legal analysis, politics, and normative argument. It sheds light on dimensions of the incident that have been obscured. And it offers a fresh perspective on ongoing debates regarding the use of force today.

The book is organized into five parts. The first three take the reader back to the nineteenth century. The fourth and fifth turn to the subsequent trajectory of international law and how we should understand Webster's formula today. It is worth considering each in turn.

Part I provides a rich and multidimensional narrative of the raid and its protagonists. For reasons that will become clear, two points stand out from a legal perspective. First, weeks before the raid, Mackenzie's insurgents (including American sympathizers) had used New York as a launchpad from which to seize and then assert governmental authority over Navy Island—a small, but secure plot of Canadian land in the Niagara, from which they then shelled populated sections of the Canadian mainland (pp. 19–21, 50). The introduction of the Caroline augmented the supply line from New York to the island, but both the cross-border insurgent attack and the contributions of U.S. nationals and U.S. territory to it were well underway by late December. The raid on the steamer was reactive, not preemptive. Although not unknown, this aspect of the affair has often been obscured, including in recent invocations of the incident.

Second, a key factor in the United States’ failure to quell its nationals’ contributions to Mackenzie's operation was the unclear and tense relationship between the objectives and authorities of the federal government, New York state, and the local population in the young republic (pp. 13–14, 36, 279 n. 32). These internal difficulties were not straightforwardly soluble. Federal enforcement depended on local militias for manpower (pp. 50–51) and President Martin van Buren was a states’ rights enthusiast (p. 60). The insurgency was not attributable to the United States, but the federal republic's structure of authority and power impaired its willingness and ability to prevent the use of its territory and the cross-border participation of its nationals.

The legacy of the raid, of course, lies less in the facts of what happened in the final weeks of 1837 than in the legal and diplomatic exchanges that followed. Part II of the book retraces those dynamics. Forcese tells a story of escalating tension that highlights the role of inflammatory reportage, combustible public opinion, and the distinct diplomatic vices of silent neglect and bluster. Yet, as he emphasizes, the early foreign affairs challenges of American federalism were also central. Wariness of war in Washington was offset by indignation at the raid in New York.

That schism was particularly pronounced on the issue of Alexander McLeod—a British-Canadian who was arrested and charged with murder for the raid's sole confirmed American fatality (Amos Durfee).Footnote ¹ New Yorkers wanted McLeod hanged. Britain insisted on his official immunity and warned that his execution would trigger war (pp. 83, 85, 112). Even when persuaded by the British position on immunity, successive American presidents held that they lacked the constitutional authority to override the New York courts (pp. 84–85, 112), which allowed the case to proceed (pp. 104–05).

It was only once McLeod's jury acquitted him (p. 114) that the conditions were apt for national economic incentives, softening politics, and the mutual personal trust of (the newly appointed) Webster and Ashburton to motivate reconciliation. Forcese's precise chronicle of this sequence offers a rich resource for students of diplomacy and federalism.

Reconciliation, however, did not mean agreement on the merits. Despite settling on a legal standard, the parties continued to disagree about who was ultimately at fault. The weight of historical opinion has not been so balanced. More than a century after the raid, Humphrey Waldock wrote that it was “commonly accepted” that the British action satisfied Webster's test.Footnote ² In his own evaluation of the merits of the case in Part III, Forcese reaches the same conclusion. More significantly, he contextualizes this assessment within the broader nineteenth century legal framework.

As is well known, the positive law of the time imposed no substantive constraint on states’ recourse to war. As such, the Caroline dispute centered on the law governing the resort to force short of war. The question was whether the raid was lawful, given that Britain purported to remain throughout in a state of peace and neutrality with respect to the United States.

Forcese's analytic clarity in grappling with the complex nineteenth century regime on force short of war is one of the book's strengths. The category of relevance to the events on the Niagara was that of the “necessity of self-preservation,” or what Stephen Neff has termed “emergency action.”Footnote ³ State practice invoking the concept was inconsistent. Britain itself had relied on an extremely permissive version of the principle to defend its 1807 assault on neutral Denmark as a necessary bulwark against the risk of a French military advance (ch. 18). However, on Forcese's telling, the parties to the Caroline dispute relied relatively little on such sources, drawing instead on the writings of the leading jurists of the seventeenth and eighteenth centuries to underpin a narrower rule of self-defense, which Ashburton termed “the first law of our nature” (pp. 119, 132–33, 138–39).

As Forcese notes, the impending dominance of positivism would soon make the diplomatic use of such a naturalist approach unthinkable (pp. 126, 165, 186). And yet, as an expression of the parties’ shared opinio juris around a moment of state practice, Webster's formula became a positive bridge between the natural law right of self-defense and the enshrinement of that “inherent” right (and not “self-preservation”) in the UN Charter a century later (pp. 127, 165, 189–90).

Nonetheless, the Caroline’s jus ad bellum prominence today is puzzling. The raid was completed in a matter of hours, caused one fatality, and triggered no direct interstate hostilities. It is not clear that in employing the language of “self-defense,” British or American representatives in the 1840s actually intended to articulate a doctrine distinct from that of “self-preservation” (p. 190).Footnote ⁴ And most importantly, as elaborated below, the legal context and function of Webster's formula differ fundamentally from the context and function of Article 51 of the UN Charter.

Forcese grapples with the Caroline’s longevity in Part IV, charting invocations of the affair through a period of profound legal change triggered by the renunciation of war in the Kellogg-Briand Pact (1928) (Pact)Footnote ⁵ and the ban on force in the UN Charter (1945). It is a journey that is likely to leave the reader with as many questions as answers. The most pressing of those is how (if at all) to make sense of the formula's transposition across radically different jus ad bellum contexts.

One theory that would appear to make sense of the doctrine's durability is that, after the renunciation of war in 1928, the only remaining legal authority for the use of interstate force was the regime governing forcible measures short of war.Footnote ⁶ On this view, far from rendering prior doctrine inapt, the ban on war elevated the relevance of Webster's formula. Forcese indicates sympathy for this perspective when he argues that the ban on war transformed the Caroline doctrine from being a “minor sideshow” (given that scaling such action up to war would render it “fully legal anyway”) to being the central and consequential legal basis for the resort to force (p. 179). Indeed, he suggests that self-defense (and by extension Webster's formula) “only became consequential” and “could matter only” once the blanket sovereign right to go to war was eliminated (id.).

For adherents to this view, the UN Charter further raised the significance of the Caroline by narrowing the scope of permissible forcible measures short of war to the “inherent right” of self-defense (p. 200). As Neff puts it, the thesis here is that “the UN Charter had the effect of forcing all instances of armed conflict into one or the other of two categories [aggression/self-defense or Security Council enforcement action]—neither of which constituted a war.”Footnote ⁷

This is a tidy account. However, it is not without difficulty. For one thing, it is in some tension with the way states and other authorities approached the issue at the time. The London Charter that provided for the International Military Tribunal at Nuremberg codified the crime of waging wars of aggression; the parties to World War II understood themselves to be at war; and the Tribunal rooted the criminality of aggression in part in the fact that it violated the Pact's renunciation of war.Footnote ⁸ None of these can be reconciled easily with the notion that a relationship of aggression and self-defense was categorically distinct from a relation of war.

Indeed, Forcese seems uncomfortable with any reliance on a strict distinction between war and non-war to make sense of the developing legal regime in the interwar period. He terms the notion that the regulation of war during that period did not extend to forcible measures short of war an “unhappy view” precisely because the formal distinction between war and measures short of war was “never particularly tenable” (p. 182).

Reconciling this view with the enduring significance of self-defense, Oona Hathaway and Scott Shapiro explain that the Pact's advocates addressed the issue of self-defense not as an instance of non-war, but by arguing that “states would be released from their obligation not to resort to war against a state that itself resorted to war [thereby violating the Pact].”Footnote ⁹ Forcese cites Quincy Wright for a similar view (p. 189).Footnote ¹⁰ Notably, the Pact's co-author, Frank Kellogg, insisted that states would retain the authority to resort “to war in self-defence.”Footnote ¹¹

Perhaps one can dismiss the references to “war” as the product of confusion.Footnote ¹² Perhaps the better view is not that aggression violated the Pact, but that the renunciation of war in the Pact stripped the legal shield (the authority to declare war) that had previously protected states and their officials when they used a certain form of devastating violence against other states and their nationals.Footnote ¹³ With that legal shield removed, the only shields remaining would be found in the law on the resort to force other than war—a legal regime that had predated the Pact and that would be codified and narrowed in 1945, in the UN Charter. On this view, it is not that aggressive war was banned in 1928, but that war authority ceased to be available as a legal defense for aggressors.

However, even if that is right, a puzzle remains. Webster's formula was developed in a context that hinged on the mutually definitive distinction between war and peace, neutrality and non-neutrality. These legal concepts require one another to be cognizable. Confining the legal category “war” to the “dumpster of international law” (p. 198), does not simply leave the law of peace as it existed prior. As the law of “non-war,” the latter makes sense only given viability of the concept of “war.”

In this sense, Forcese's assertion that self-defense doctrines “only really became consequential” with the ban on war (p. 179) is not altogether convincing. As he makes very clear, Webster's formula had a purpose when it was crafted. It attempted to define the scope of force that would neither wrong the affected state nor entail the “calamitous rupture in diplomatic relations” of war (p. 171). The formula would also be compatible with an ongoing mutual posture of peace and neutrality (pp. 148, 175). The very fact that the diplomatic exchanges around the Caroline dispute were so tense and lengthy indicates that these were not insignificant stakes.

To be sure, those stakes differ fundamentally from what is at stake in a self-defense doctrine that also regulates war (or, to put it differently, a doctrine that exists in the absence of a separate and unchecked right to go to war). But that is precisely the point. Prohibiting war changed what the doctrine of self-defense entailed in ways that call into question the enduring relevance of a version of the doctrine developed before those changes. It is for that reason that the ongoing invocation of the Caroline has been derided in some quarters as “far-fetched,” “anachronistic,” and “misguided.”Footnote ¹⁴

The point here is not that war ought to be normatively exceptional.Footnote ¹⁵ Rather, the point is that such an exceptionalism was enshrined in the positive law of the time of the Caroline raid and was fundamental to what was at stake in the diplomatic exchange that followed. Of course, it is not Forcese's job to make conceptual sense of how Webster's formula has survived these transformations. The formula may simply have provided a useful tool at a moment in which various actors were desperate for a doctrine of self-defense, regardless of its conceptual aptness (pp. 186–90). However, it is surprising that Forcese, having both detailed the fundamental legal transformations and engaged the conceptual question regarding the Caroline’s survival of them, does not grapple more comprehensively with the challenge of anachronism.

It is worth pausing here to note that despite the elimination of war as a legal category, the Caroline context resonates (imperfectly) with some ways of framing contemporary debates regarding the purportedly defensive use of force against non-state actors abroad. Specifically, some scholars claim that such uses of force can be limited so as neither to infringe the rights of, nor to create a state of armed conflict with, the host state.Footnote ¹⁶ To take one example of a state asserting such a view, in 2001, Iran declared that a “limited and proportionate” defensive military operation against non-state armed actors in Iraq “should not be construed as infringing the territorial integrity of Iraq” and claimed to “look[] forward to promoting friendly relations with its neighbour.”Footnote ¹⁷ However, such lines of argument are controversial.Footnote ¹⁸ Moreover, whatever their merits, they are far from the only scenarios in which the Caroline affair has been invoked in the UN Charter era.

Setting to one side the issue of conceptual aptness across jus ad bellum contexts, the empirical question of what has sustained the Caroline over time remains. For Forcese, a significant part of the answer lies in academia. Among others, he cites the roles of Robert Phillimore (who had a personal interest in the raid) and Lassa Oppenheim in bringing the incident to prominence early on in their leading international law treatises (pp. 173–77), the impact of a flurry of academic work in revitalizing the incident around its centenary (p. 189–90), and the import of the writings of Humphrey Waldock, Rosalyn Higgins, Robert Jennings, and Phillip Jessup in entrenching the Caroline in international law consciousness in the UN Charter era (p. 205). Academic voices did not merely raise the profile of the correspondence. Forcese charges both Phillimore and Oppenheim with disseminating misleading or distorted summaries of the raid particularly by reframing it as an anticipatory, rather than reactive action (pp. 176–77, 229), an error that was then entrenched by the International Military Tribunal at Nuremberg (itself relying on misleading secondary sources) and repeated so frequently since that some continue to frame the incident as being exclusively about preemption (pp. 204, 227–29, 263).

Quite apart from spotlighting the trajectory of that particular error, Forcese's chronicle of the affair's enduring prominence also reveals the power of key treatise writers (particularly of general texts on public international law) in defining which events matter and how. In several respects, the Caroline is particularly ripe for reflection on that point.

First, its legal function is the product of a remarkable cycle between practice and theory. As noted above, Forcese identifies the influence of the eminent naturalist jurists of the seventeenth and eighteenth centuries on the diplomats developing British and American opinio juris in response to the events of December 1837. The formula those diplomats agreed was then given a sustaining level of prominence by the positivist academics of the nineteenth and twentieth centuries, who in turn made it accessible and ripe for the state and judicial practitioners of the post-Charter era. Indeed, Forcese observes that in the UN Charter era, the likes of Waldock, Jennings, and Arthur Watts revised the Brierly and Oppenheim treatises to change the categorization of the Caroline dispute from self-preservation to self-defense, thereby maximizing the viability of Webster's formula in that changed context (p. 205).

Second, the influence of treatise writers in shaping how historical events are translated into legal doctrine by states, courts, and others (as exemplified by the trajectory of Webster's formula) raises important questions about authority in the evolution of law. The repetition of error spotlights the phenomenon, but the issue is not merely one of accuracy; it also matters who selects and frames the key incidents and cases that shape our legal imaginations. The most prominent and cited Anglophone treatise writers and academic commentators are based disproportionately in the United Kingdom or the United States. This raises the possibility that the Caroline’s prominence is at least in part a consequence of the academic hegemony of the leading institutions of the states involved in the dispute, where there is naturally a parochial interest in it, rather than being the result of the incident's inherent universal significance.

Forcese argues that Webster's formula “has escaped its history and its Anglo-American origins,” noting its status as a “litmus test” of self-defense not just in leading treatises, but in a “massive body of literature” (p. 206). However, his supporting endnotes on this point are telling. They cite 581 U.S. law review articles and a study by former British Foreign Office lawyer Elizabeth Wilmshurst under the auspices of Chatham House (p. 334 nn. 18–19).

Moreover, while the incident has no doubt also gained traction in a broader transnational literature and has been invoked by a diverse range of states in formal pleadings (pp.193–94, 336 n. 26),Footnote ¹⁹ the key question is what explains that result. It is notable, for example, that the Caroline had featured prominently in the academic writings of several British and American International Court of Justice judges before they ascended to the Court (p. 205). The subsequent reflection of their editorial judgment in states’ pleadings may reflect litigation strategy as much as it does substantive buy-in.

Third, and conversely, it is impossible in the aftermath of Anthea Roberts's towering work on the divisions within international legal discourse not to question whether the apparent ubiquity of the Caroline may be an illusion underpinned by a focus on anglophone sources.Footnote ²⁰ Notably on this front, Michael Wood claims that “virtually every author writing in the field of self-defence refers to the Caroline incident” before offering the caveat that this does not hold in France, where the affair tends to be ignored.Footnote ²¹ It seems unlikely that France is truly the only holdout in this respect.Footnote ²² If a multilingual analysis were to dilute the perceived influence of Webster's formula, this would raise interesting questions regarding how it ought to be used going forward and whether its current status will diminish as the distribution of global power shifts.

Part V of the book turns from the durability of Webster's formula to its three substantive elements—necessity, proportionality, and imminence. Plainly, Webster did not invent these principles (p. 165). Nonetheless, the jus ad bellum literature continues to trace them to his letter.Footnote ²³ Omitted from the text of the UN Charter, the elements have been interpreted to be definitive features of the “inherent right” it enshrines.

Formally, necessity and proportionality impose criteria additional to responsiveness to an armed attack, thereby adding layers of constraint to the Charter's text. Imminence, on the other hand, might be thought to loosen the Article 51 rule, which (by its text) allows force only when an armed attack “occurs.”

In practice, however, the role played by these elements is less clear-cut. Forcese notes, for example, that necessity has been used both to bolster the case for preemptive self-defense (on the grounds that it is the only way to quell an attack) and the case for a loose interpretation of imminence within the framework of preemption (on the grounds that the last window for preventing an attack may long pre-date it) (ch. 28).Footnote ²⁴ Such reasoning transforms necessity from a constraint to an enabler, situating it as the normative keystone of self-defense, rather than as an element of it. Imminence, in contrast, is given only derivative significance; its additional and distinct epistemic function erased.Footnote ²⁵

The difficulty with the proportionality criterion inheres in its substantive ambiguity. On one view, jus ad bellum proportionality demands that states use the least harmful means to achieve the legitimate defensive objective.Footnote ²⁶ Others have argued that there must be some proportion between the harm threatened (or inflicted) by the attack (or, alternatively, the defensive objective) and the defensive force used.Footnote ²⁷ Kretzmer calls this a “narrow proportionality” assessment, which is distinct from, though sometimes confused with, the kind of tit-for-tat proportionality characteristic of reprisals. Footnote ²⁸

For Forcese, these are the two alternative interpretations of jus ad bellum proportionality between which we must adjudicate (pp. 235–36). Overlooked in this assessment is a third interpretation—namely, that, like its analogue in human rights law, jus ad bellum proportionality requires both that force be limited to that necessary to achieve the defensive aim and that the overall harm inflicted by that force not be excessive in relation to the legitimate defensive aim it pursues (or possibly the threat to which it responds).Footnote ²⁹ Moreover, debates over what counts in a proportionality analysis have layers beyond that topline question. Consider, for example, the topical issues of whether (and how) to continue to evaluate jus ad bellum proportionality in an ongoing conflictFootnote ³⁰ and whether the defensive aim incorporated into the calculation should include only direct prevention or also longer-term objectives such as deterrence.Footnote ³¹

One might think that the depth of ongoing disagreement on the meaning of imminence, necessity, and proportionality limits the utility of Webster's formula as a contribution to the jus ad bellum today. For Forcese, however, their malleability is a strength. He argues that it makes for a regime that is “capable of bending without snapping,” granting the powerful sufficient room not to dismiss it out of hand, while still serving as “grit on a quick slide to war” (pp. 244–49). Moreover, he contends, the result is more affirming of the rule of law than would be the likely more frequent “recourse to ‘illegal but legitimate’ justifications” under a more restrictive regime (p. 245).

This line of reasoning hinges in part on the claim that Webster's elements in fact perform the role of “grit on a quick slide.” In tentative support of this assertion, Forcese points to the global reduction in territorial conquests identified by Hathaway and Shapiro in The Internationalists, and claims further that Webster's elements slowed and increased the cost of the invasion of Iraq in 2003, as well as informing Barack Obama's decision not to use force against the Syrian regime following a chemical attack within Syria in 2013 (pp. 246–49).Footnote ³²

This is not altogether convincing. Whatever one's views on the frequency of territorial conquest as the relevant measure in this realm,Footnote ³³ much more would be needed to attribute that decline to the elements of the Caroline formula. Indeed, Hathaway and Shapiro themselves attribute the result not to Webster's elements, but to the elimination of the pre-Pact sovereign authority to use war as a mechanism to resolve disputes and to gain compensation in the form of recognized annexed territory.Footnote ³⁴ Although it is not inconceivable that proportionality (for example) has played a role too, one would need to offer a theory of that impact, and then test it against the available information to make such a claim credibly.

The anecdotal examples of Iraq and Syria are no more persuasive. Whatever grit Webster's formula placed on the quick slide, Iraq was invaded in 2003, despite significant political opposition, a woefully weak legal case, and the subsequent determination by the Iraq Inquiry in the United Kingdom that the “circumstances in which it was decided that there was a legal basis for UK military action were far from satisfactory.”Footnote ³⁵ In the case of Syria, the action did not go forward, but the legal difficulties were not rooted in Webster's elements. The core international law problem for the Obama administration in 2013 (which was anyway likely not the key factor in his administration's decision not to use force) was that the proposed action would have entailed a humanitarian reprisal, not self-defense, or even humanitarian intervention.Footnote ³⁶

Moreover, American inaction in that case was a rarity. As Forcese acknowledges, the United States has for years been engaged in protracted and non-consensual armed conflict on Syrian territory, as part of its broader and seemingly endless against a shifting conglomeration of non-state groups across multiple states (p. 241). It is with respect to conflicts of the latter kind (which are those that most closely approximate the context of the Caroline) that concern has been most pointed regarding ambiguity in the definitions (and expansive interpretations) of necessity, proportionality, and imminence.

On this issue, Forcese considers the possibility that the “rediscovery of the Caroline’s facts” could help us to grapple with the problem arising from the “serious incongruity” of allowing non-state armed groups to “hide behind the territorial sovereignty” of states unable or unwilling to prevent their transnational activity (pp. 239–42). Specifically, he argues for “terrorist-specific targeting,” and against a “one-way ratchet that cannot be notched back” (pp. 241–42). Such a limited approach could be facilitated, on his account, if the host were to take the “wise” position of treating “the ‘preventive’ presence of foreign forces as friendly rather than as hostile acts” (p. 242).

Needless to say, this is an optimistic analysis and Forcese recognizes that, in practice, the legacy of Webster's formula on this front is ambiguous. Indeed, it may well have helped to underpin the very one-way ratchet that he fears. Moreover, even if that were not the case, the notion of “terrorist-specific targeting” is a dangerous formulation that glosses over both the rights of those deemed “terrorists,” and the physical and mental impact of such strikes on the surrounding population and territory.Footnote ³⁷ Even if that were the contemporary legacy of the affair, it would warrant close critical scrutiny.

Overall, Destroying the Caroline is essential reading for jus ad bellum scholars. It broadens and deepens our understanding of what has become a central feature of our jus ad bellum discourse. It provides the foundation for critical reflection and debate on the curious and winding route through which the incident gained this status and on the utility of Webster's formula today. And it addresses many of the central challenges in the jus ad bellum today in a thoughtful and provocative way.