Leggi in ItalianoOona Hathaway and Scott Shapiro’s The Internationalists advances a thesis that is simple to state and surprisingly hard to refute: the legal prohibition of war, beginning with the Pact of Paris in 1928, transformed the entire structure of international relations. Not gradually, and not merely symbolically — but as the single hinge on which a whole system of rules turned.
I came to the book expecting a conventional history of the interwar period. What I found was something more ambitious: an argument that every rule of the old international order — conquest, coerced treaties, neutrality, the legal immunity of belligerents — rested on one premise, namely that war was a lawful instrument. Once that premise was removed, the entire edifice had to be rebuilt.
In the system that Grotius articulated and that prevailed from roughly 1600 to 1928, war functioned as the international equivalent of a lawsuit. A state that had been wronged and could not obtain redress by negotiation was entitled to enforce its rights by force. The consequences followed logically: territory taken in war belonged to the victor. Treaties signed under duress were valid. Soldiers fighting on either side had a legal licence to kill. And third states were bound to remain neutral — to treat both belligerents alike, which meant that economic sanctions by outsiders were not merely unusual but illegal.
Hathaway and Shapiro document this system with striking precision. They compiled more than 400 war manifestos published between 1492 and the Second World War, cataloguing the reasons sovereigns gave for going to war: self-defence (69 per cent), breach of treaty (47 per cent), compensation for injury (42 per cent). The pattern is revealing: war required justification. It was not anarchy; it was a legal procedure.
The turning point came from an unlikely quarter. Salmon Levinson, a Chicago business lawyer with no diplomatic credentials, proposed in 1918 to do something that seemed absurd: outlaw war itself. His analogy was duelling. Centuries of rules on how to conduct a duel had never eliminated duelling; what ended it was declaring the act itself criminal. The Pact of Paris, signed on 27 August 1928 by fifteen states and eventually joined by sixty-three, adopted this logic. Its two operative articles are remarkably brief: the parties renounce war as an instrument of national policy and agree to settle disputes by peaceful means. There is no enforcement mechanism and no definition of self-defence.
What happened next is the core of the book’s argument. A treaty with no teeth turned out to have consequences that reshaped the world. If war was illegal, then conquest could no longer produce valid title. When Japan seized Manchuria in 1931, the United States declared — through what became known as the Stimson Doctrine — that it would not recognise any territorial change achieved by force. The League followed. For Hathaway and Shapiro, this is the moment the right of conquest died in international law. If war was illegal, the old neutrality obligation made no sense either: third states were no longer required to treat aggressor and victim equally. Step by step through the 1930s and 1940s, the United States moved from strict neutrality to open assistance to one side, eventually reaching Lend-Lease. Economic sanctions became lawful precisely because war had become unlawful. And if war was illegal, those who started one had no legal immunity. At Nuremberg, the crime against peace was grounded in the Pact. The argument was that the Pact did not create a new crime: it removed an old privilege.
The UN Charter of 1945 completed the transformation. Article 2(4) banned not just “war” but any “threat or use of force,” closing the loophole Japan had exploited in Manchuria by claiming it was not technically at war. Force remained lawful in only two circumstances: self-defence against armed attack and collective action authorised by the Security Council.
The data Hathaway and Shapiro present are among the most compelling elements of the book. Between 1816 and 1928, there were on average 1.21 conquests per year, with roughly 295,000 square kilometres of territory changing hands annually. Between 1949 and 2014, the figures dropped to 0.26 conquests per year and about 15,000 square kilometres. Before 1928, a state could expect to lose territory by conquest once in a human lifetime. After 1948, the probability fell to once or twice in a millennium. The authors accept other explanations — nuclear weapons, democracy, trade — but argue that these all assume the more fundamental point: that might no longer makes right.
The new order has a price. When conquest was legal, weak states were absorbed and the number of states steadily declined. Once the right of conquest disappeared, small states could survive and colonies could become independent without fear of a new master. The UN grew from 51 members to 193. Some of these states are fragile, and some have failed, becoming the ground from which armed groups — from al-Qaeda to the Islamic State — have operated. The prohibition on force protects every state, including those that cannot govern themselves. Without war as an enforcement mechanism, international law relies on what the authors call “outcasting”: excluding rule-breakers from the benefits of cooperation. The WTO authorises trade retaliation; sanctions regimes target states that violate international norms. Outcasting has real limits — it does not work against states already isolated, and it favours the powerful.
The book’s data stop in 2014. Since then, Russia’s invasion of Ukraine and its claimed annexation of four Ukrainian regions have tested the thesis directly. The refusal of most of the international community to recognise those annexations, and the unprecedented scope of the sanctions imposed on Russia, suggest that the system the authors describe is still operating. Whether it will prove sufficient is another question — and one that connects the book’s historical argument to the most pressing legal and political debates of our time.