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War is illegal.

Yet the biggest single change in the international order is an idea we seldom appreciate today: war is illegal. For most of history, that was not the case. Might made right, war was the continuation of policy by other means, and to the victor went the spoils. If one country felt it had been wronged by another, it could declare war, conquer some territory as compensation, and expect the annexation to be recognized by the rest of the world. The reason that Arizona, California, Colorado, Nevada, New Mexico, and Utah are American states is that in 1846 the United States conquered them from Mexico in a war over unpaid debts. That cannot happen today: the world’s nations have committed themselves to not waging war except in self-defense or with the approval of the United Nations Security Council. States are immortal, borders are grandfathered in, and any country that indulges in a war of conquest can expect opprobrium, not acquiescence, from the rest.

The legal scholars Oona Hathaway and Scott Shapiro argue that it’s the outlawry of war that deserves much of the credit for the Long Peace. The idea that nations should agree to make war illegal was proposed by Kant in 1795. It was first agreed upon in the much-ridiculed 1928 Pact of Paris, also known as the Kellogg-Briand pact, but really became effective only with the founding of the United Nations in 1945. Since then, the conquest taboo has occasionally been enforced with a military response, such as when an international coalition reversed Iraq’s conquest of Kuwait in 1990–91. More often the prohibition has functioned as a norm—“War is something that civilized nations just don’t do”—backed by economic sanctions and symbolic punishments. Those penalties are effective to the extent that nations value their standing in the international community—a reminder of why we should cherish and strengthen that community in the face of threats from populist nationalism today.

Steven Pinker. 2018. Enlightenment Now