The book begins with a bleak description of the ‘Old World Order’, which rested on the right of states, in the absence of a world court, to resort to war to redress grievances or solve disputes. War was a legal mechanism. Hathaway and Shapiro’s study of more than four hundred declarations of war from the late 16th century to 1939 reveals that self-defence and the enforcement of treaty, international or succession laws were the reasons cited most often by states. In addition to permitting frequent armed conflict, the lawful status of war had other consequences for international relations. Since force could be used to resolve conflicts, the system rewarded the powerful, sanctifying the principle of ‘might is right’. It also legitimated conquest, both as compensation for injury and as the outcome of a contest of force in which the weaker side lost. It permitted the threat of force (gunboat diplomacy). It protected the decision makers who waged war and the soldiers who fought it, because both were engaged in a legal activity. Killing in war wasn’t murder. And, finally, lawful war required absolute impartiality from neutrals (for example, in their trade or commerce with belligerents), since they were not parties to the dispute. Economic sanctions were therefore illegal. This state of affairs lasted into the 20th century, and Hathaway and Shapiro see the First World War as its ‘terrible culmination’. Even the League of Nations ‘did not herald’ its end because its covenant still permitted member states to resort to war over serious, non-judiciable disputes after a three-month cooling-off period.

Hathaway and Shapiro’s premise is that since states seemed incapable of weaning themselves off warfare, civil society had to intervene. They focus on four ‘internationalists’ who helped broker, institutionalise and interpret the Kellogg-Briand Pact. The first is Salmon Levinson, a corporate lawyer from Chicago whose pamphlet The Legal Status of War (1918) argued forcefully that all wars except those begun in genuine self-defence should be criminalised, and all conquests declared null and void. In six months, Levinson printed 350,000 copies of the pamphlet, helping to fire up public and elite opinion. James T. Shotwell, a historian at Columbia, more conventionally targeted only ‘aggressive war’, defined as force used after (and in spite of) mandatory arbitration and judgment from the Permanent Court of International Justice. This definition led to a strengthening of the League of Nations’ rules: it made the determination of ‘aggression’ automatic, removing it from the vicissitudes of voting by members of the League or the spurious claims of individual states. The ‘Geneva Protocol’ of 1924 containing Shotwell’s definition of ‘aggressive war’ was ratified by 47 member states, but its salutary effect applied only to them. Shotwell consequently began to adopt Levinson’s more sweeping language, calling for war to be outlawed altogether. Behind the scenes, these two internationalists sought to influence government leaders. A Shotwell memorandum was the basis for Briand’s overtures towards the United States, and a draft treaty drawn up by Levinson outlawing war reached Kellogg in the State Department.

The diplomatic horse-trading that led up to the Paris Peace Pact seems contrary to the nobility of its expressed aims. Since the Allied victory in 1918, the US had retreated into isolationism, Britain had turned its attention to safeguarding its empire, the Soviet Union was immersed in revolution, and Germany was bent on revising the Treaty of Versailles. France had been left to uphold the postwar order virtually alone. At first, it relied on reparations and military force – alliances with Czechoslovakia, Romania and Yugoslavia (the Little Entente), and its occupation of the Ruhr in 1923 in order to force Germany’s timely repayment of reparations. The Ruhr occupation almost bankrupted France; shortly after, its foreign policy, led by the multilateralist Briand, began to move in the direction of legal rather than military guarantees of French and European security.

In 1927, Briand, using Shotwell’s memorandum, publicly called on the US and France to agree on ‘“the outlawry of war”, to use an American expression’. His object was to obtain a bilateral treaty of neutrality with the US that might be a first step in re-engaging America in Europe, ending France’s isolation from its former allies and bolstering its security. Kellogg was keen to wriggle out of any such responsibility. The US had rejected membership of the League of Nations at least in part to avoid involving itself in European conflicts. But Briand’s proposal that France and the US mutually renounce war ‘as an instrument of national policy’ struck a chord with the public in both countries. Kellogg realised he could satisfy opinion (and enhance his international reputation) without abandoning isolationism if the agreement were general in its language, devoid of enforcement mechanisms and universal, rather than bilateral.

The resulting pact fulfilled these requirements. Diplomatic exchanges before the signing seemed to weaken it even more. In an effort to escape the trap of bilateral obligation, Kellogg solicited Britain’s views and then invited other nations to join in signing the treaty. In the course of negotiations, Kellogg recognised France’s demand that wars started in self-defence or out of obligation to allies were still lawful; Britain defined self-defence as applying to the empire in its entirety; and the US reaffirmed the Monroe Doctrine. These were big loopholes – big enough that the pact could be dismissed by James Reed, the Democratic senator from Missouri at the time, as ‘nothing but an international kiss’.

Nevertheless, or perhaps on this account, 63 nations had signed by 1934 – about 85 per cent of the world’s states. Both France and Britain, though fearful of the potential impact on their security, signed because of pressure from the peace, feminist, trade union and internationalist movements, as well as a desire not to offend the powerful US, but also because there was genuine enthusiasm among politicians and civil servants to extend international laws. Germany, limited by the Versailles Treaty to an army of a hundred thousand, was the first to sign because an instrument limiting war actually increased its relative power and promised peaceful revision of the hated treaty. Japan, however, believed it was merely making a ‘diplomatic gesture … affirming the aspiration of all civilised nations to seek peace’.

Hathaway and Shapiro argue that none of this mattered. The Kellogg-Briand Pact became international law and took on a life of its own. It affected US foreign policy when the secretary of state Henry L. Stimson withheld recognition of Japan’s conquest of Manchuria in 1931 (the ‘Stimson Doctrine’), and Italy’s of Ethiopia in 1936. After the US entered the Second World War, Sumner Welles, the third internationalist featured in the book, successfully pressed President Roosevelt to proclaim the pact’s principles as Allied war aims in the Atlantic Charter of 1 January 1942, which called on ‘all nations of the world’ to ‘come to the abandonment of the use of force’.

The fourth figure they discuss is Hersch Lauterpacht, originally from Galicia but by 1937 Whewell Professor of International Law at Cambridge, who rigorously and successfully argued that the Kellogg-Briand Pact had overturned the basic structures of the international order. Neutrals were no longer bound to impartiality, permitting policies that helped victims of aggression, like the Lend-Lease Act of 1941, by means of which the US provided military aid to countries ‘deemed vital to the defence of the United States’. Because it resulted from a criminal act, conquest was now illegal. Individual leaders could be held responsible for waging illegal wars (the principle behind the Nuremberg Trials). And treaties extorted by coercion were invalid. Lauterpacht’s briefs to the US and British governments in the 1940s helped establish these principles, making him ‘the father of the New World Order’.

Since 1945 that order has been characterised by remarkably few inter-state wars or annexations. In diplomacy sanctions have mostly replaced the threat of war. But Hathaway and Shapiro are well aware that war hasn’t disappeared, but migrated and changed form. It now takes place mostly within states, as the result of decolonisation, civil strife or failed regimes. Some of these wars, as in Syria, are spectacular in terms of the deaths, displacement and international disruption they have caused, but none compares in intensity with the world wars. Hathaway and Shapiro’s point is that ‘for all its problems, the New World Order is better than the Old.’ Theirs is a valuable reminder that law matters and that international co-operation is not a utopia, but a functioning reality. Recently, it has been hard to hear that truth above the din produced by bad actors, like Putin and Trump, and by criticism of the neoliberal order from the left and the populist right, which obscures the positive effects of internationalism. What’s more, we take for granted a world in which the assumption is that countries will not engage in war.

In order to make their case Hathaway and Shapiro have to argue starkly and their account tends to simplify the long and uneven history of legal change. As an ‘intellectual history’, The Internationalists seems to argue that ideas and thinkers are the main determinants of law. It presents legal change as precipitate, with little or no preparation in state practice or world opinion. We never learn the antecedents of Levinson, Shotwell or Welles’s ideas, which are presented as wholly antithetical to everything that preceded them.

If ideas, here, seem to make law by themselves, law is made to seem almost magical in its transformative capacities. The authors write that ‘legal revolutions do not end with the passing of a law. They begin with them,’ by which they mean that legal and political institutions, economics and other structural factors develop in response to a legal framework and then tend to reify it. Most people would agree that law is a fundamental force shaping international society, and that analyses which leave out its influence are misleading and incomplete. But The Internationalists has the opposite fault, treating law as if its explanatory power were almost unlimited, as for example when the huge growth in the number of civil wars since 1945 is attributed to the fact that ‘these conflicts are not prohibited by the pact.’

It is possible to conceive of Kellogg-Briand differently, as the product of a long, halting and uneven process by which European states tried to limit war among themselves. Even the 17th-century Dutch lawyer Hugo Grotius, described by the authors as ‘the father of International Law’, restricted lawful war to states and prohibited it (following the precepts of the ‘just war’ tradition) if undertaken for the sake of plunder, mere advantage, better land, newly discovered title, universal religion, even independence in cases where subjection resulted ‘from lawful causes’. More important, the states themselves gradually whittled down the accepted justifications for war. The Thirty Years War impelled them to remove religion as an allowable motive and to set up political and judicial means of adjudicating disputes between Roman Catholics, Lutherans and Calvinists in Central Europe via the Aulic Council or Imperial Chamber Court.

The settlement of the Napoleonic Wars was another watershed. The Congress of Vienna in 1815 fixed the borders of Europe, guaranteed the right to existence of small and medium-sized states and established the holding of frequent congresses and conferences to settle conflicts before they mutated into wars. Post-Napoleonic statesmen focused on preventing ‘aggressive war’, the tradition that Shotwell worked in and Levinson rejected in favour of a total ban. The concept of aggressive war has remained central to the development of international law and attempts to restrict war. It was the Allied analysis of Germany’s role in starting the First World War, and it was the charge made against Germany and its government in the Armistice and the Treaty of Versailles. It was forbidden to League of Nations members under Article Ten. It was again the Allied analysis of Nazi Germany and Imperial Japan’s wars, and was therefore the foundation for the indictments in the Nuremberg Charter. It is one of the four basic crimes under the jurisdiction of the International Criminal Court (ICC). For decades, states have struggled to define aggression as a legal crime. Finally in 2010 at the Kampala Review Conference, they agreed that aggression would mean ‘the planning, preparation, initiation or execution’ by a person in effective authority of an act ‘which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations’. As of 17 July this year the ICC may begin to hear cases alleging that crime.

Other methods were also employed in the hope of reducing armed conflict. Arbitration as an alternative to war was used increasingly over the 19th century as states concluded bilateral treaties obliging them to submit disputes to arbitration. In 1899 the Permanent Court of Arbitration was established at The Hague Conference (it still operates). So widespread was the expectation that states would refer disputes to it that Germany and Austria-Hungary’s failure to do so in 1914 was the primary reason that Allied and neutral opinion held them responsible for starting the war. Governments talked about arms control, and although attempts at this before 1914 were ineffective, they added to the weight of influential opinion seeking to make inter-state wars less likely. The International Committee of the Red Cross, founded in 1863, was one of many early NGOs that successfully educated and mobilised public opinion. States also developed systems of ‘pacific blockade’ that used economic pressure to settle disputes short of war (as in 1860, Sardinia against Sicily; 1862, Britain against Brazil; or 1886, five European nations against Greece). In 1907 at The Hague the use of force to collect state debts was prohibited.

After the First World War, the pace quickened. Kellogg-Briand was preceded by a host of often unrealised attempts to make it more difficult to wage war: the Covenant of the League of Nations; the draft Treaty of Mutual Assistance (1923), which made aggressive war a crime; the abortive Geneva Protocol, which narrowed the definition of lawful war; the resolution of the September 1927 League of Nations Assembly declaring the use of war to settle disputes ‘an international crime’; the Locarno Treaty of 1925 forbidding its signatories from resorting to war; and a wave of bilateral treaties doing the same.

This developing consensus helps explain how law becomes law – that is, obligatory, not wishful – and why most states are inclined to follow it. The two world wars can be interpreted as global struggles between the states that wanted to anchor the Kellogg-Briand principles, to turn them into institutions and practices, and those that either rejected those principles outright or were happy to see them remain ‘pious aspiration’, in a phrase of the time.

There is no inevitable march of progress in history or law. Everything that has been achieved can be rescinded, forgotten, tossed away. That is the message Hathaway and Shapiro want to convey. It seems aimed primarily at Americans, whose current president is the chief denigrator of international institutions. They may be forgiven for exaggerating the role of the US in outlawing war and in fashioning the institutions that sustain the hope of international co-operation. They claim, for example, that ‘a great American president [Roosevelt] risked his life’ to make the agreement founding the United Nations possible (by going to Yalta when desperately ill), and the agreement was ‘fundamentally an American document – conceived by Americans, negotiated by Americans, and made possible by Americans’. The heroes of this book are the four internationalists, three Americans and one naturalised Briton; the anti-heroes, besides Grotius, are German (Carl Schmitt), Japanese (Nishi Amane) and Egyptian (Sayyid Qutb, the intellectual inspiration for al-Qaida and Islamic State – an idiosyncratic choice). The Internationalists has shortcomings, but it is a timely and necessary plea for international law and for the value of institutions from which we all have benefited, but which we have in recent decades neglected to explain or defend. The unavoidable conclusion is that we must stop lamenting and get up and do something.

Isabel Hull teaches at Cornell. A Scrap of Paper: Breaking and Making International Law During the Great War was published in 2014, and reviewed in the LRB by Susan Pedersen

London Review of Books