What happened to a 1928 pact to "outlaw war"?

War became illegal in 1928 under the Kellogg–Briand, signed by the US, Germany and France. Given the 20th century’s subsequent bloody history, this might surprise you, as it did some observers at the time. Signed with a foot-long golden fountain pen, the Pact was mocked as “the international equivalent of an air kiss.” Ridicule has since been replaced with indifference. But this new book by Oona Hathaway and Scott Shapiro argues the Pact is “among the most transformative events of human history.”

Before 1928, “might was right.” When Plenty Horses shot Lieutenant Ned Casey, in 1891, he walked free from court having proved that the Sioux were at war with the US. The Pact replaced this “licence to kill” with a demand to co-operate. It made the world more peaceful by requiring states to seek alternatives to conflict, such as economic sanctions, and it encouraged free trade: the lure of conquest was reduced if a state no longer required huge territories to gain access to markets.

But wars still happen; in the absence of conquest, weak states are allowed to survive and may become failed states, vulnerable to civil war and terrorism. The authors conclude with a passionate argument against Donald Trump’s brand of populism and protectionism: free trade and global co-operation have kept wars at bay.

For a work laden with intellectual and legal history, this is surprisingly colourful, filled with personalities and anecdotes, delivered in a chatty tone. Its lightness of touch is only matched by its richness of content, packing a lot into 600 pages.

The Internationalists—and their Plan to Outlaw War is by Oona A Hathaway and Scott J Shapiro (Allen Lane, £30)