The Internationalists: How a Radical Plan to Outlaw War Remade the World. By Oona Hathaway and Scott Shapiro. Simon & Schuster; 608 pages; $30. Allen Lane; £30.

THE rules-based international order that emerged from the wreckage of the second world war was a huge improvement on any preceding era. It stimulated trade on an unprecedented scale and allowed even relatively small and weak countries to develop their potential without fear of predatory interference. At the heart of that order was an underlying principle that perpetrators of aggressive war should not be rewarded. In particular, any territorial gains which derived from their aggression would not be recognised by the international community as being legitimate. Instead, aggressors should be subjected to punishment—usually economic sanctions. Occasionally, concerted military action approved by the United Nations (UN) forced them to relinquish what they had illegally seized.

Yet liberal internationalism is now under attack from many sides. Donald Trump’s America First doctrine explicitly repudiates it. Even two of the so-called “adults in the room”, who supposedly temper Mr Trump’s nativist excesses, seem happy to join him. In a Wall Street Journal article in May H.R. McMaster and Gary Cohn, respectively the president’s national security adviser and economic adviser, wrote: “The world is not a ‘global community’ but an arena where nations, non-governmental actors and businesses engage and compete for advantage. We bring to this forum unmatched military, political, economic, cultural and moral strength. Rather than deny this elemental nature of international affairs, we embrace it.”

Apart from the bit about cultural and moral strength, neither Vladimir Putin nor Xi Jinping, who both challenge the liberal international order by seeking to create spheres of influence through intimidation and military bullying, would find anything to disagree with in that statement. Mr Putin annexed Crimea in 2014 (the first time that the borders of post-war Europe had been changed by force) and launched a covert invasion of eastern Ukraine in support of a separatist insurrection. Mr Xi is attempting to make the South China Sea, through which over half the world’s commercial shipping passes, into a Chinese lake by creating artificial islands in defiance of international law.

“The Internationalists” by Oona Hathaway and Scott Shapiro, both law professors at Yale, is an impassioned history of how the liberal international order came into being and why it must be defended as never before. They believe that the basis of what they call the New World Order (to distinguish it from the Old World Order, codified by a 17th-century Dutch scholar, Hugo Grotius, in which might was nearly always right) was an extraordinary diplomatic event in Paris in 1928. The General Treaty for the Renunciation of War as an Instrument of National Policy, or the Kellogg-Briand pact (named after the foreign ministers of the United States and France who had sponsored it, pictured seated right and left), was signed by more than 50 countries, including all the great powers.

The pact was a direct consequence of the “Great War” of 1914-18—a truly Grotian conflict that had left 11m combatants dead. Its purpose was to outlaw aggressive war and territorial conquest. But there was a problem of enforcement. Japan’s seizure of Manchuria in 1931 was never legitimised, as it would have been under the Old World Order, but a new system had not yet come into effect which could make Japan surrender its prize. Neither the signatories of the pact nor the League of Nations was willing or able to stem the rise of militarism during the decade that followed and its apotheosis in the second world war.

The ideas underpinning the pact did, however, have a profound influence on the way in which the allies saw both their fight against the Axis powers and the organisation of the peace that followed. When the war ended, with the partial exception of the Soviet Union, the victors handed back the land they had conquered. The Nuremberg trials re-established the principle that waging aggressive war was a criminal act and punished at least some of Hitler’s henchmen accordingly. The founding of the UN and the establishment of the International Court of Justice in The Hague, although far from perfect, have had very positive effects. Gunboat diplomacy imposed by major powers on weaker countries became an anachronism. So too did interstate war between them.

Of course there are still plenty of wars. In some ways the New World Order, which has helped make international wars so much less imaginable, has inadvertently made possible more “intranational” wars. Fragile and fractious countries that would previously have feared being conquered by more powerful neighbours can now fall prey to civil wars or brutal insurgencies without bad actors fearing loss of the national territory they seek to control. Non-state groups, such as Islamic State (a misnomer), can take and hold, at least for a while, territory from dysfunctional governments. Well-meaning but ill-conceived wars to change odious regimes have sometimes gone badly wrong. Foreign-policy realists will also, with justification, point out that the main reason why great powers no longer fight each other is because the destructive force of nuclear weapons has removed any incentive to do so.

Yet the authors argue persuasively that the liberal order of the past 70 years has been better than any of the alternatives and is well worth striving to preserve. The authors pay proper tribute to those who defined and fought for the principles that brought it into being. They include Salmon Levinson, a Chicago lawyer whose ideas led directly to the Kellogg-Briand pact; Sumner Welles, an American diplomat who envisaged the creation of a world organisation with the military clout to bring future warmongers to book; Hersch Lauterpacht, a great Polish-British jurist who helped create a body of international law based on universal values and human decency; and James Shotwell, a Canadian academic who worked with Aristide Briand to bring the pact into being and later contributed to the design of the UN.

Ms Hathaway and Mr Shapiro are right to sound the alarm that the post-second-world-war consensus on the illegality of war is under siege. Among those threats are militant jihadism; an angry Russia and an ambitious China determined to challenge an international system they believe fails to reflect their interests; Iranian support for terrorist groups; and North Korea’s contemptuous dismissal of diplomatic attempts to rein in its nuclear programme. But perhaps the greatest danger at present is the incumbency of an American president who despises international norms, who disparages free trade and who continually flirts with abandoning America’s essential role in maintaining the global legal order. The “internationalists”—the heroes of this important book—must be spinning in their graves.