International Law and Its Transformation Through the Outlawry of War

Oona A Hathaway & Scott J Shapiro. International Affairs. Volume 95, Issue 1. January 2019.

The First World War was the last great war of what we have called the ‘old world order’—the legal regime that European states adopted in the seventeenth century and spent the next three centuries imposing on the rest of the globe. This order formed the basis of what scholars call ‘classical international law’. But this body of rules differed starkly from the ones that govern today: the old world order did not just sanction war, it relied on and rewarded it. States were permitted to wage war to right any legal wrong, and the right of the victors to extract territory and treasure from the losers was legally guaranteed. That all began to change when the nations of the world decided to outlaw war in the 1928 Kellogg–Briand treaty. As a result, the rules governing international behaviour have transformed radically—indeed, they are the polar opposite of what they once were. This article describes the decision to outlaw war and the transformation it unleashed in the world order generally, and in international law specifically. We argue that a simple but perplexing fact—that modern international law prohibits states from using force to enforce international law—is key to understanding international law and state behavior in the modern era.

The outbreak of the First World War was unremarkable from a legal point of view. It started with a legal wrong. On Sunday morning, 28 June 1914, Bosnian Serb Gavrilo Princip assassinated Archduke Franz Ferdinand and his wife Sophie. Austria-Hungary responded to this wrong by issuing an ultimatum to Serbia to investigate the plot behind the assassination and prosecute the conspirators. Serbia agreed to meet all but one of the demands. To Austria-Hungary, however, less than complete compliance was unacceptable and, on 28 July, it issued a declaration of war.

Russia sided with Serbia and began mobilizing its troops. Germany took Russian mobilization as a prelude to an attack and, asserting its legal right of self-defence, declared war on Russia. Russia responded by declaring war on Germany. Germany asked France whether it planned to abide by the Franco-Russian military alliance. When no assurance of neutrality was forthcoming, Germany declared war on France as well.

Germany asked Belgium for permission to march through its territory so that it could attack France. Belgium refused, citing the Treaty of London of 1839, which mandated its neutrality. When Germany ignored Belgium’s refusal, Great Britain—a signatory to the 1839 treaty—declared war on Germany. Austria-Hungary responded by declaring war on Britain and France. Britain and France returned the favour. Italy and Japan also sided with France, Russia and Britain; the Ottoman empire and Bulgaria took the other side. The United States stayed out of the fray until 6 April 1917, when, citing Germany’s U-boat campaign as a violation of the legal rules of neutrality and freedom of the seas, it too declared war.

On the surface, the conclusion to the First World War was similarly unremarkable. To the victors went the spoils. Britain and France demanded indemnification for the costs that Germany had imposed on them, just as Germany had demanded 5 billion francs in gold as indemnification at the end of the Franco-Prussian War of 1871. The victors also carved up the territory of the losing states. Thus, the Ottoman empire was dissolved and much of its territory parcelled out to Italy, France and Britain. The successor state of Turkey retained only a fraction of its prewar empire. Austria-Hungary was also dismembered; its successor states of Austria and Hungary lost substantial territory to Italy, Poland, Czechoslovakia, Romania and the Kingdom of the Serbs, Croats and Slovenes. Bulgaria was also reduced, with parts of its territory going to Greece and the Kingdom of the Serbs, Croats and Slovenes. Areas of German land were given to Belgium, France, Poland and Portugal. Most significantly from the French perspective, Alsace-Lorraine—land conquered by France in 1648 and then by Germany in 1871—was returned to France.

In short, the First World War was the last great war of what we have called the old world order—the legal regime that European states adopted in the seventeenth century and spent the next three centuries imposing on the rest of the globe. This order formed the basis of what we now call ‘international law’; but that law differed starkly from the law that governs today. The old world order did not just sanction war, it relied on and rewarded it. States were permitted to wage war to right any legal wrong, and the right of the victors to extract territory and treasure from the losers was legally unassailable.

We argue that the decision of the nations of the world to outlaw war in the 1928 Kellogg–Briand treaty triggered a change in international law and, as a result, in state behaviour. This argument is antithetical to realist thought, which holds that a change in the law cannot possibly bring about a change in state behaviour; only a change in incentives can do that. As we have written elsewhere, that reaction misunderstands how law works. When law is most effective, it does not induce states to act contrary to incentives; it changes those incentives themselves. Our approach bears some similarities to constructivist work in that we, like most constructivists, see key aspects of international relations as historically and socially constructed. But our work is distinct from much constructivist work in self-consciously locating the role of law in the process of transformation.

In the century since the close of the First World War, the rules governing international behaviour have changed radically—indeed, they are now the polar opposite of what they once were. This article draws and builds on the argument made in our book The internationalists: how a radical plan to outlaw war remade the world to tease out the implications for international law of the decision to outlaw war. It begins by describing the old world order. Then it describes the decision to outlaw war and the transformation it unleashed in the world order generally and in international law specifically. Finally, it considers the consequences of this transformation for international law today. We argue that a simple but perplexing fact—that international law prohibits states from using force to enforce international law—is key to understanding international law in the modern era.

The Old World Order

To understand how much has changed in the international legal order, it is necessary first to understand what the legal order once was. The old world order was defined first and foremost by the belief that war is a legitimate means of righting wrongs. The inhabitants of the old world order would have found the famous maxim from Carl von Clausewitz’s On war—’War is simply the continuation of politics by other means’—to be incontrovertibly true. Resorting to arms did not signal a failure in the system: it was how the system worked. War was an instrument of justice. Might was right.

This principle was put succinctly by the great international lawyer Hugo Grotius in 1625: ‘Where judicial settlement fails, war begins.’ States were legally permitted to go to war, in other words, because, in the absence of a court with the power to give them relief, they had no other option. For centuries, states operating under this principle used war to enforce their rights. They fought to collect debts, to recover tort damages, to protect trading rights and to enforce treaty obligations, among other reasons.

In the old world order, the legality of war as a tool of justice gave rise to a range of other legal rights. To begin with, all states had the right of conquest: any state that claimed it had been wronged by another state, and whose demands for reparations were ignored, could retaliate with force and capture territory as compensation. The conquering state thereby became the new sovereign of the captured territory: it owned all public property and possessed the legal authority to rule over its subjects. Nearly every border in the world today bears witness to some such past battle—including that of the United States. Arizona, California, Nevada, Utah, and parts of Colorado, New Mexico and Wyoming, are no longer part of Mexico because the United States launched a war in 1846 over unpaid debts.

Not only did states have the legal right to wage war to redress wrongs, they could also threaten to wage war for the same purpose. When Japan refused to trade with the United States in the nineteenth century, violating its obligation to participate in global commerce, the United States sent Commodore Matthew Perry with a fleet of gunboats to offer a ‘treaty of friendship’. He left the Japanese in no doubt that the alternative to friendship was war. The old world order also granted immunities to those who waged war—in effect, authorizing mass homicide. If an ordinary person killed another outside war, it was a murderous crime. If an army killed thousands during a war, it was not only lawful but glorious. Those who waged war could not be held criminally responsible for doing so.

While waging war was legal, economic sanctions by neutrals against belligerents were prohibited. A state that favoured one side over another in an ongoing war could be punished, even if it never fired a shot. Thus, if a neutral state traded with a belligerent but refused to trade with its opponent (or traded, but on less favourable terms), it violated its duty of neutrality and could be attacked in retaliation. Had the United States traded with Britain but refused to trade with Germany when the First World War began, it would have violated its duty of neutrality and Germany would have been entitled to attack.

The old world order was not simply a collection of individual rules; those rules fit together in a coherent and cohesive system. Once states had a privilege to use force to impose their rights, a range of legal rules inevitably followed. If war was legal, conquests won in war had to be legal—a permissible form of compensation for legal wrongs. If states could wage war to right wrongs, they could also threaten to wage war for the same purpose. They could, moreover, enforce any agreements that resulted from such duress—with war, if necessary. If war was legal, then those who waged aggressive war had to be immune from prosecution. And if war was legal, then any neutral state that sought to punish a state that waged aggressive war by putting in place economic sanctions was violating its duty of neutrality—and that was a just cause of war. These rules all stemmed from the permission to use war to right wrongs, and they supported and reinforced one another.

At the conclusion of the First World War, the old world order was under strain. The victors did not merely demand indemnification for costs they had incurred as the result of the war. They demanded reparations. Article 231 of the Versailles Treaty stated:

The Allied and Associated Governments affirm and Germany accepts the responsibility of Germany and her allies for causing all the loss and damage to which the Allied and Associated Governments and their nationals have been subjected as a consequence of the war imposed upon them by the aggression of Germany and her allies.

As a result, the following article 232 demanded that ‘Germany make compensation for all damages done to the Civilian Populations of Allied and Associated Powers … by such aggression’.

Though article 231 came to be known as the ‘war guilt’ provision, it did not use the word ‘guilt’. Nevertheless, the provision did assign responsibility for the war to Germany and the assignment was total: Germany had to accept ‘responsibility … for causing all the loss and damage‘. In making this stipulation, the Treaty of Versailles broke with past practice. Until Versailles, as Randall Lesaffer has shown, European peace treaties persistently shied away from moral or legal attributions. To put the past behind them, victors did not blame the losers or assign to them responsibility for precipitating the war. Monetary costs were often imposed, but they were not styled as ‘reparations’. Thus, when Germany demanded money from France in 1871, the transfer was characterized merely as a ‘payment’ (‘La France paiera à S.M. l’Empereur d’Allemagne la somme de 5 milliards de francs’). The victors thought of their demands as compensation for wrongs committed but, in the interests of peace, they dared not recriminate.

The unprecedented moralization of war also appeared in article 227 of the Treaty of Versailles: ‘The Allied and Associated Powers publicly arraign Wilhelm II of Hohenzollern, formerly German Emperor, for a supreme offence against international morality and the sanctity of treaties.’ Never before had a European sovereign been held liable for starting an aggressive war. Contrast that article with, for example, article 3 of the 1814 Treaty of Fontainebleau, concluded after the defeat of Napoleon: ‘The island of Elba, adopted by his Majesty the Emperor Napoleon, for the place of his residence, shall form, during his life, a separate principality, which shall be possessed by him in all sovereignty and property.’ Napoleon was not arraigned for waging war on most of Europe for two decades. Instead, he was given his own island to rule, becoming the Emperor of Elba.

But even as the victors in the First World War strained against the rules of the old world order, that order’s essential legal commitments held. The longstanding rule against criminal prosecution of sovereigns and combatants for waging war, which both followed from and enabled the legal right of war, was challenged but ultimately prevailed: the German Kaiser was indicted for launching the war, but the Netherlands refused to hand him over for trial. There could be no criminal prosecution of a leader for merely waging aggressive war. The novelty of the effort to prosecute a leader for waging war is evident from the treaty text itself. Article 227 states that Wilhelm had committed an offence against ‘international morality’ and the ‘sanctity of treaties’ and that a future tribunal should be guided ‘by the highest motives of international policy’. It did not say that he had committed a crime—because he had not.

Even the League of Nations stands as evidence of the continuing power of the old world order. To prevent another global war, the League of Nations sought to resolve disputes through compulsory dispute resolution. Articles 13 and 15 of the Covenant required member states, instead of proceeding to war, to submit their disputes to the League: either to an arbitral tribunal agreed to by the parties, the new Permanent Court of International Justice, or to an ‘inquiry’ by the League Council. Having done so, states were obliged to wait until one of these bodies reached a judgment. If a state won the dispute, and the loser complied with the judgment, the winner could not go to war. But if a state lost and did not want to comply with the judgment, article 12 permitted it to resort to war, provided it waited three months before doing so. In order to enforce this system of dispute resolution, the Covenant created a mechanism of collective coercion. Article 16 stated that members that went to war in contravention of the Covenant would face sanctions determined by the council—including not only trade and financial sanctions but potentially military sanctions as well.

This introduction of compulsory enforcement by neutral states on belligerents was a legal novelty. Since the eighteenth century, neutrals were subject to a strict duty of impartiality: as noted above, non-belligerent states were legally forbidden from granting privileges to one belligerent and not its enemy. A state that favoured one side in an ongoing war wronged the other by interfering with that state’s exercise of its right of war. This wrong was itself a just cause, entitling the victim state to respond with force—even if the neutral state never fired a shot. It was for this reason that President Woodrow Wilson, who ran for re-election in 1916 on the slogan ‘He kept us out of war’, called on Americans to remain ‘impartial in thought as well as in action’.

Before the creation of the League, then, economic sanctions by neutrals on belligerents had been illegal. Indeed, political scientists who study the history of economic sanctions have identified no cases of economic sanctions outside war before the First World War. By establishing an international organization capable of enforcing compliance through sanctions—financial as well as military—the peacemakers had broken with the past.

But even though the League of Nations aimed to contain the right of war, it did not deny it. States that took their disputes to the League, went through its prescribed process and waited three months were within their legal rights to wage war. In constraining war but preserving states’ ultimate right to resort to it, the peacemakers in Paris were following a long development in European international law that sought to reduce the horrors of armed conflict, while still preserving the right to go to war. The First Geneva Convention in 1864 had protected wounded and medical personnel; the St Petersburg Declaration of 1874 had prohibited the use of fragmenting, explosive and incendiary small arms ammunition; the First Hague Convention of 1899 had banned explosives from balloons, asphyxiating gas and dum-dum bullets; the Second Hague Convention of 1907 had proscribed pillage, the execution of surrendering soldiers and prisoners of war, and forcing civilians to swear allegiance to a foreign power. The League’s use of arbitration followed the First Hague Convention’s establishment of the Permanent Court of Arbitration.

Despite all these changes, the right of war remained intact: states were entitled, as a last resort, to use force to right wrongs done to them. And yet the peace that emerged from Versailles showed signs that states were uncomfortable with the system of international law which had produced the Great War. The imposition of reparations on Germany, the attempt to prosecute the Kaiser for the crime of war, and the collective enforcement through economic sanctions all suggested a profound change in attitude. In the League Covenant, the peacemakers affirmed the right of war, but exposed their discomfort with its implications.

When 29 nations gathered in Paris in January 1919, they came not only to conclude the war but to seek an end to the system of international relations that had led to the senseless bloodbath. In the words of the British diplomat Harold Nicolson: ‘We were journeying to Paris, not merely to liquidate the war, but to found a new order in Europe. We were preparing not Peace only, but Eternal Peace.’ That effort failed. Indeed, it is one of the great ironies of the First World War that a catastrophe which had killed 1.8 million Germans, 1.7 million Russians, 1.4 million French, 1.3 million from Austria-Hungary and nearly 1 million from the British empire, could not bring an end to the right of war.

The Outlawry of War

The First World War did, however, set in motion a chain of events that would transform the international legal order and eventually end its dependence on war. Even before hostilities ended, anti-war activists were taking aim at the foundational principle of international law: the legal right to wage war. As Salmon Levinson, one of the leaders of the so-called ‘Outlawry of War’ movement, put the point in August 1917: ‘The real disease of the world is the legality and availability of war.’ The aim of the Outlawry movement was to make the waging of war illegal. ‘We should have, not as now, laws of war, but laws against war; just as there are no laws of murder or of poisoning, but laws against them.’

Levinson and the organization he founded to promote his idea, the American Committee for the Outlawry of War, tapped into an already robust peace movement, made up of hundreds of loosely coordinated groups, many of them formed during or shortly after the First World War. Jane Addams, a Chicagoan like Levinson, had founded the Women’s International League for Peace in 1919 and served as its first president. The League organized chapters not only in Washington DC and New York, but across the country and around the world, providing a network for events in support of outlawry.

The first target of Levinson and his allies was the League of Nations, which he saw as a creature of old world order principles. Rather than bring peace, he argued, it would perpetuate the legality of war. In one of the many undated memos to himself that he was in the habit of writing (perhaps meant as a draft of an article or letter), Levinson explained his opposition to the League by likening it to antiquated practices of medieval hygiene:

Recently I heard a man cite a statement from a book on sanitation in the middle ages to the effect that in the days before bath tubs had been invented perfumes were used very profusely, and that when bath tubs came in, perfumes very largely went out. Now, our international experts are sold on perfumes, so to speak. They think to get rid of war’s menace by stifling its stench somewhat. No matter how poor a perfume is put on the market, they never fail to embrace it eagerly nor to give it the most flattering advance notices, especially if it has been bottled in a certain town in Switzerland.

The League Covenant negotiated by President Woodrow Wilson was just more perfume masking the rankness of war. Outlawry would be the cleansing bath. ‘To outlaw war means to abolish this now lawful institution by smashing its legal props and branding it a crime.’

Several members of the Senate, whose approval was required in order for the United States to join the Versailles Treaty, agreed. Their fears focused in particular on provisions of the proposed Covenant—particularly articles 10 and 11—that appeared to require members to come to one another’s aid in the event of an act of aggression. Senator Henry Cabot Lodge, then chairman of the Senate Foreign Relations Committee, objected to participating in an organization that might draw the United States into ‘internal conflicts in other countries, no matter what those conflicts may be’. Lodge proposed adopting the Covenant with reservations intended to clarify that the United States would not go to war without congressional approval. President Wilson dismissed the proposal as an attempt to undermine the agreement he had so carefully crafted. He lost Lodge’s support and with it, though he did not realize it at the time, probably any hope of gaining Senate approval.

The Republican Senator William Borah, who served on the Senate Foreign Relations Committee (and would later become its chair), agreed with Lodge. He wanted nothing to do with an international organization that could order the United States to impose sanctions or go to war. ‘You may still delude yourself, as others have done in the past, with appearances and symbols, but when you shall have committed this Republic to a scheme of world control based upon force,’ Borah warned from the well of the Senate, ‘you will have soon destroyed the atmosphere of freedom.’ Borah objected to the extraordinary demands that the League was prepared to impose—including the requirement to go to war against an aggressor—in order to enforce its rules. Led by Borah and Lodge, the Senate rejected the Versailles Treaty.

The next step for Levinson and his allies was to devise a plan for peace based on outlawry. In 1919, Levinson and Senator Philander Chase Knox worked together on a pamphlet—entitled Plan to outlaw war—that would explain outlawry to the members of Congress and the public. It would take nearly a decade, but eventually the audacious proposal to outlaw war resulted in a treaty. On 27 August 1928, 15 foreign ministers gathered together at the Quai d’Orsay in Paris to sign the ‘General Treaty for the Renunciation of War’, better known today as the Kellogg–Briand Pact. In article 1 of this document, states renounced their right of war: ‘The High Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another.’ Within a year (the treaty entered into force in 1929), virtually every state in the world had ratified the pact and thereby renounced their right of war.

Contrary to Levinson’s expectations, of course, the pact did not cure the ‘disease’ of war. Just three years after the grand signing ceremony in Paris, Japan invaded China. Four years after that, Italy invaded Ethiopia. Four years after that, Germany invaded Poland and then most of Europe. Soon thereafter—with the exception of Ireland—every one of the states that had gathered in Paris to renounce war was at war.

The Transformation Set in Motion by the Pact

The League of Nations is remembered as a failure, but the Kellogg–Briand Pact is hardly even remembered. Most historians do not discuss it. There is not one reference to the pact in either The Penguin history of the world or Oxford’s History of the world, each over 1,200 pages long.

To the extent that the pact is mentioned, the reference is almost always derisive. Robert H. Ferrell thought it evinced the ‘appalling’ naivety of the American public. Hans J. Morgenthau questioned whether it was anything more than ‘a statement of moral principle without legal effect’. Ian Kershaw described it as ‘singularly vacuous’. Henry Kissinger mocked the outlawry of war as ‘irresistible as it was meaningless’. George Kennan called the very idea behind the pact ‘childish, just childish’.

It is wrong, however, to conclude from the lamentable fact that the 1928 pact did not bring about world peace that outlawing war was ineffectual. Contrary to the received wisdom, the outlawry of war set in motion a transformation in the international legal order. The new legal order did not emerge fully formed along with the pact; indeed, it would take more than a decade to build, piece by painstaking piece.

The failure of the authors of the Kellogg–Briand Pact to figure out how a global order grounded in the rejection of war could function, given the world’s long reliance on war, became clear almost immediately. When, in September 1931, Japan invaded Manchuria, the League was paralysed. After all, nearly all its members had just renounced war. The prohibition on war certainly could not be enforced with war. But if not with war, then with what?

The answer came from Henry Stimson, the American Secretary of State and successor to Frank Kellogg. Stimson had been a classmate of Levinson’s at Yale College. Levinson had written an article for the Christian Century and the editor had sent it to Stimson. In the article, Levinson argued that the outlawry of war must obliterate the right of conquest. ‘If it is unlawful to wage war, conquests by war should furnish no legal title.’ War might still be waged, but it would no longer work as before, for it would mean that ‘never again can a nation bent upon conquest acquire indefeasible title to anything’. The refusal to recognize conquests would act as a real sanction against would-be conquerors.

Stimson drafted a diplomatic note and, on 8 January 1932, a copy of it was sent to the governments of China and Japan. The note set out Levinson’s policy of non-recognition—what would later come to be called the ‘Stimson Doctrine’: ‘The American Government … does not intend to recognize any situation, treaty or agreement which may be brought about by means contrary to the covenants and obligations of the pact of Paris of August 27, 1928.’ The League of Nations quickly followed suit, calling on its member states not to recognize any conquest ‘brought about by means contrary to the Covenant of the League of Nations or to the pact of Paris’.

Though Wilson had come to Paris in 1919 to end conquest and annexation, his creation did not succeed in ending this practice because the League Covenant preserved the foundational right of war, which in turn legitimated conquest. The point of war, after all, was to right wrongs, and the seizure of land was the standard form of compensation. It would take the outlawry of war in 1928 to end the legal recognition of conquest. As long as there was a right of war, there had to be a right of conquest. Only once that right was renounced could the right of conquest truly be challenged.

It was not long before international lawyers and politicians began to rethink other rights that had been connected to the legal right to resort to war. In 1934, an international assemblage of scholars in Budapest concluded a report on ‘The effect of the Briand–Kellogg Pact of Paris on international law’. In addition to accepting the Stimson Doctrine, it concluded that the law of neutrality ceased to protect states that had violated the pact. Neutrals were now allowed to discriminate between belligerents, including selling munitions to victims, providing them with financial assistance, and refusing to allow aggressors the right to visit and search their vessels.

The League had imposed economic sanctions on Japan for its conquest of Manchuria in 1932, but those sanctions proved to be ineffective—not because they were too weak, but because they were too strong. By cutting off Manchuria from the world, the world had cut itself off from Manchuria. Even China, the country which had most to benefit from sanctions, found that sanctions interfered with its ability to conduct foreign trade. Thus, when Italy invaded Abyssinia, the League was reluctant to inflict substantial sanctions on a country in the heart of Europe. It thus imposed token trade restrictions on Italy—in the words of a British diplomat, ‘a series of half-way measures, measures intensely exasperating to the Italians without being effective’.

The United States’ reaction was even more limited. While it did not recognize the seizure of Abyssinia, neither did it impose economic sanctions on Italy. Not only did imposing sanctions seem futile, but many international lawyers in the United States still believed that such actions were illegal—as they always had been. Instead, the US imposed a ‘moral embargo’, in which Americans were encouraged to boycott Italian goods, and the President slapped an arms embargo on both countries, as required by the Neutrality Act.

The dispute over neutrality came to a head after Great Britain and France declared war on Germany in 1939. Far from keeping the United States out of the war, the country’s neutral stance towards the belligerents made its entry into the European war more likely: for if Germany defeated Britain, the United States would almost certainly have to fight overseas. President Franklin Delano Roosevelt was therefore desperate to supply urgently needed war materiel to Prime Minister Winston Churchill. One of the strongest opponents to providing support to Britain was Roosevelt’s own Secretary of War, Harry Woodring. When Woodring would not bend, Roosevelt pressed him to resign, replacing him with none other than Henry Stimson (a surprising choice, not only because of Stimson’s advanced age, but also because he was a lifelong staunch Republican). With Stimson in office, the administration began pushing the bounds of traditional neutrality aggressively. On 2 September 1940, the United States traded 50 retired destroyers to the United Kingdom for the right to establish bases on British territory.

Eager to increase American assistance even further, Roosevelt pushed for, and in March 1941 signed, the Lend-Lease Act, which allowed the United States to discriminate between belligerent nations when it sold, lent or disposed of supplies. In the first 90 days after the act was passed, the United States allocated US$4.25 billion of authorized aid, immediately made available 2 million gross tons of cargo ships and oil tankers to carry the aid, and began training 7,000 British pilots.

To justify these actions, extraordinary for a neutral state, the Attorney General Robert Jackson gave a speech at the Inter-American Bar Association in Havana on 27 March 1941 laying out the administration’s legal reasoning. Having been counselled by the great international lawyer Hersch Lauterpacht, Jackson claimed that traditional rules of neutrality requiring strict impartiality were based on the idea that ‘all wars are legal’. However, the outlawry of war had changed all that. ‘The Kellogg–Briand Pact of 1928’, he argued, ‘of necessity altered the dependent concept of neutral obligations.’ Neutral states were now permitted to aid one belligerent rather than another without violating any legal duties. He concluded his speech with a stirring call to use the pact as the foundation for a new world order. ‘The principle that war as an instrument of national policy is outlawed must be the starting point in any plan of international reconstruction. And one of the promising directions for legal development is to supply whatever we may of sanction to make renunciation of war a living principle of our society.’

With Jackson’s speech, the transformation of the world order that had begun in 1928 took yet another step towards completion. Now, not only was conquest, once legal, illegal, as the Stimson Doctrine made clear; neutral states were no longer required to remain impartial, clearing the way for economic sanctions to become a new tool of international law enforcement. The other two pillars of the old world order—gunboat diplomacy and impunity against criminal prosecution for aggression—were also on the brink of collapse. Once waging aggressive war was no longer permitted, threatening to wage wars to obtain enforceable promises could no longer be allowed either.

Duress became recognized as a defence to any effort to enforce a treaty negotiated at gunpoint—a right enshrined after the Second World War in the Vienna Convention on the Law of Treaties. The end of that war also brought the Nuremberg Trials, and with them a definitive declaration that there would be no more impunity for waging aggressive war. When war was legal, those who waged it could not be held criminally responsible. But now that war was illegal, the world declared that waging aggressive war was not only wrong—it was criminal.

International Law in the New World Order

Many consider the United Nations simply a reboot of the League of Nations. After all, even the names of the key institutions are nearly the same—the ‘League of Nations’ was replaced by the ‘United Nations’; the ‘Permanent Court of International Justice’ was replaced by the ‘International Court of Justice’; the ‘Assembly’ was replaced by the ‘General Assembly’; and the ‘Council’ was replaced by the ‘Security Council’. The key difference, according to this view, is that the United States did not make the same mistake in the wake of the Second World War that it made in the wake of the First World War. In the 1940s, instead of retreating from the global community and refusing to join the international organization its president had conceived, the United States took on leadership of the community and championed the organization—not only joining the United Nations, but hosting the conference at which it was inaugurated (in San Francisco in 1945) and housing it in New York City, where it remains today.

That story, however, fails to recognize a deep dissimilarity between the two organizations. It is not the engagement of the United States that marks the crucial difference between the two (though that was undoubtedly essential to the UN’s success). The fundamental difference is in fact that the League of Nations was a creature of the old world order, and as such retained the right of states to resort to war at its core. As we have seen, the Covenant merely attempted to curtail the right, to slow down its exercise and to put peaceful dispute resolution in its place. But ultimately it allowed states to do what they had long done: go to war to enforce their rights as they saw them.

The United Nations, by contrast, is best understood as the culmination of the outlawry vision. Its authors placed the prohibition on the ‘use of force’ at its core: article 2 of the UN Charter prohibits states from resorting to the ‘threat or use of force’ against another state. The Charter then created an institutional structure around this prohibition to maintain international peace and security.

The new world order is not without its challenges. States cannot now act—legally—unilaterally to address violence in other states unless they have been attacked or face imminent attack (or have been asked to assist another state facing such a threat). The prohibition on use of force applies regardless of whether the target state is a model democracy or a crushing autocracy, a strong or a weak state, or even a failing one. This gives rise to a dilemma. The rules of the new world order that provide so much benefit protect all states from the use of force, including those we do not want to protect because they are too weak, chaotic, authoritarian or, for lack of a better word, evil.

To see the difficulty that would arise from giving states the option to use force when they believed they were acting justly, one need only look at Syria and Ukraine, two places where the rules against force have broken down. The United States has declared that it regards the regime of Syrian President Bashar al-Assad as illegitimate and that it believes there is no future for his government in Syria, even as Russia has sent in troops and weapons to protect the regime from collapse. In Ukraine, the circumstances are reversed: the United States has supported the government against aggressive Russian-backed separatists determined to overthrow the revolutionary government that unseated President Yanukovych.

The UN Charter has a mechanism for overriding the universal protection provided by article 2—a vote by the UN Security Council. But the Council has been hamstrung by the very disagreements just described. In both the cases outlined above, the five permanent members have been unable to agree to override the prohibition on the use of force and authorize intervention. Thus the background prohibition remains: no state may use force without violating the most fundamental rule of the system, the prohibition on war.

The challenge is even greater than this suggests. For the prohibition on war not only prevents states from intervening to protect the rights of others unless the Security Council agrees to authorize an intervention or a state requests assistance defending itself from armed attack; it also prohibits states from using force to protect their own rights (except in cases of self-defence). States can refuse to join treaties with other states. After all, as explained earlier, gunboat diplomacy is no longer allowed. Once states join, they might even refuse to comply. As opposed to the old world order, where a violation of international law could trigger a military response, the outlawry of war no longer permits states to take unilateral decisions to wage war to right wrongs. In short, international law prohibits states from using force to enforce international law unilaterally.

This simple but perplexing fact—that international law prohibits states from using force to enforce international law on their own—is key to understanding international law in the modern era. In what may seem a cruel irony, the human rights revolution began just at the moment international law could no longer be implemented with force. The 16 major universal human rights treaties all postdate the creation of the UN. All of the regional human rights treaties are similarly recent. The only significant human rights treaty that predates the new world order is the 1926 Slavery Convention, which came into effect on 9 March 1927.

The emergence of international legal human rights protections at the very moment they could no longer be enforced with war is unlikely to be coincidental, however. In an era when international law—including treaties—could be enforced through war, states were understandably reluctant to enter into treaty commitments that they were not confident they could meet. They certainly would not enter into such an agreement that offered no substantial, tangible, direct benefits.

The idea of going to war over a treaty violation might seem fanciful, but it was once commonplace. We can see this by examining ‘war manifestos’—documents that set out the legal reasons sovereigns provided for going to war from the late fifteenth century through to the mid-twentieth. Those manifestos reveal that violations of international law were a common cause of war. Nearly half of the war manifestos we collected and analysed cited treaty violations as just causes for war. In addition, 42.5 per cent of these war manifestos cited the laws of war, the law of nations or customary international law. In such a world, a state would be foolish to join a treaty that committed it, for example, to protecting the civil and political rights of its own citizens in broad and general terms, as the 1976 International Covenant on Civil and Political Rights does. A violation would be inevitable—and so would war.

After the Kellogg–Briand Pact outlawed war, and after that principle was reiterated and reinforced in the UN Charter, such treaties were no longer so dangerous. A state could enter into a human rights treaty with little fear that war would follow. States were also newly free to create and ratify a wealth of environmental treaties, which were equally difficult to honour—requiring, as they often did, states to change the behaviour of third parties. Across the spectrum, the number of treaties and international organizations exploded. The latest edition of the United Nations Treaty Series includes hundreds of thousands of international agreements in force in the world, comprising over 2,900 volumes, and the Union of International Associations maintains records on over 75,000 international organizations.

Are these all simply empty promises? Doesn’t a prohibition on enforcing treaties with force create an insuperable barrier to effective international law?

It turns out that the system works much more often than this description of the situation would seem to suggest—including at some times when it would seem most likely to fail. For in the new world order, states have developed a rich set of tools to replace war as a way of enforcing international law—tools we call outcasting.

Outcasting occurs when a group denies those who break its rules the benefits available to the rest of the group. ‘Outcasting’ is non-violent: instead of doing something to the rule-breakers, outcasters refuse to do something with the rule-breakers. An example may help illustrate the point. When a state violates the rules of the World Trade Organization, no troops arrive to enforce the legal obligations. Instead, if a state breaks the rules—by, for example, putting in place an illegal tariff—a state harmed by that illegal tariff can file a complaint and prosecute its case before a tribunal (called the Dispute Settlement Body). If this tribunal rules in its favour, the WTO authorizes the state that filed the complaint to break the rules in return. That is, the WTO entitles the victorious party to suspend the benefits of membership in the community in respect of the offender. Thus, if the tribunal finds that Mexico imposed an illegal tariff on Peru, and Mexico does not lift the tariff, Peru will be authorized to impose an otherwise illegal trade barrier of equal value on Mexico. The rules are enforced not with war, but with the loss of cooperative benefits (here lower trade barriers) to which the state would otherwise be entitled.

There are hundreds of similar examples throughout international law, from the International Coffee Organization, which can kick bad actors out of the association, to the Convention on International Trade in Endangered Species of Wild Fauna and Flora, which prohibits members from trading in endangered species unless all parties to the trade follow the rules designed to protect these species. Outcasting, then, is a victim of its own success. It is so ubiquitous and so often effective that it is usually invisible. When is the last time that the evening news reported on a trade war that did not erupt, or that mail was delivered on time? Outcasting is the Holmesian dog that doesn’t bark.

Much of this outcasting would have been illegal at the time of the First World War. While war was legal under the old world order, economic sanctions by neutrals against belligerents were illegal. In an address to Congress upon the outbreak of the war in Europe, President Woodrow Wilson declared that the country would remain neutral. Wilson understood the rules of the old world order. He cautioned the public that the country could enjoy the rights of neutrality only so long as it observed its responsibilities:

The United States must be neutral in fact, as well as in name, during these days that are to try men’s souls. We must be impartial in thought, as well as action, must put a curb upon our sentiments, as well as upon every transaction that might be construed as a preference of one party to the struggle before another.

Any sign of partiality, however slight, could give the warring parties just cause to draw the country into the European inferno.

The ubiquitous use of outcasting in the modern era was made possible by the change in the law of neutrality triggered by the Kellogg–Briand Pact. As Robert Jackson declared in a speech delivered in 1941 to explain the United States’ new position on the law of neutrality, the Pact ‘did not impose upon the signatories the duty of discriminating against an aggressor, but it conferred upon them the right to act in that manner’. The shift from prohibiting to permitting discrimination meant that states that had once been required to remain impartial could now distinguish between belligerents at war with one another. Doing so was no longer a violation of neutrality, no longer a just cause for war.

Today, human rights treaties may not be enforced with war. But they can be enforced with outcasting, though doing so requires careful institutional design. Consider the European Convention on Human Rights. This is the most ambitious—and most successful—international human rights regime in the world. The Convention does not use the same direct form of outcasting as the WTO—after all, states cannot commit human rights abuses in retaliation for a state’s commission of a human rights abuse. Instead, it uses what we have called cross-countermeasures—taking away another benefit in return for the failure of a state to live up to its commitment under the treaty. A state that violates the Convention can be sued in the European Court of Human Rights. If the Court finds a violation, the defendant state must obey its judgment. If it fails to do so, it may be cast out of the Council of Europe—effectively denying it the benefits it enjoys from participating in the web of economic, political and legal ties with other member states. The strength of the human rights treaty thus ultimately rests on a threat of ejection from the Council of Europe, and a complete loss of the benefits that come with that membership—including a vast array of political, economic and regulatory programmes, and access to over 200 treaties open only to Council of Europe members.


The world order in which we now live is a photo negative of the old world order that held sway at the close of the First World War. The old world order had rules governing neutrality, criminal liability, conquest and gunboat diplomacy. The new world order that governs today has rules for all these, too—but they are precisely the opposite. In the new world order, aggressive wars are illegal. And because aggressive wars are illegal, states no longer have the right to conquer other states; waging an aggressive war is a grave crime; gunboat diplomacy is no longer legitimate; and economic sanctions are not only legal, but the standard way in which international law is enforced.

The leaders who assembled in Paris a century ago had hoped to ‘liquidate war’. Clearly, they did not succeed. But their generation was successful in greatly reducing interstate war. In a process that began in 1928, the international community decided to outlaw war as a method of enforcing rights, except in cases of self-defence. It would take decades, a world war, the creation of international institutions and the development of new forms of outcasting to bring it to fruition, but outlawry has made the world more peaceful and prosperous as result.

Our generation, however, is slowly undoing this historic process. The institutions and achievements of the post-Second World War legal order are under assault today as never before. The very countries that failed at Versailles, but succeeded in Paris, Yalta, San Francisco and Bretton Woods, are the ones now placing the international order they created under the greatest stress. Russia seized and incorporated Crimea—the first successful conquest in Europe since the Second World War. China has occupied contested territory in the South China Sea, where it has hastily built military installations, and has ignored an international arbitral tribunal decision declaring its claims illegal. France, the United States and the United Kingdom all carried out illegal military strikes in Syria in April 2018. And US President Donald Trump, who campaigned in 2016 on a slogan of ‘America First’, has time and again treated international institutions not only as nuisances but as outright threats. Whether the world has learned anything in the century since the close of the First World War will be seen in how it responds to these threats to the global legal order in the years to come.