From Vox: How war became a crime

what's the difference?

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. . . in the 20th century, and especially after the end of World War II, something changed. “Violent state death has dramatically declined since 1945 in particular,” Fazal told me. “Attempts to take specific territory haven’t been declining the same way, but successful conquests against smaller state territories have been declining.”

“Dramatic decline” isn’t the same as nonexistence, of course. Violent state death still happens; North Vietnam conquering the South in 1975 probably qualifies. All the same, as Fazal writes in her book, “State death has changed dramatically, with voluntary unifications (à la Germany and Yemen) and [voluntary] dissolutions (à la the Soviet Union and Czechoslovakia) far outpacing the rate of violent state death.”

Fazal gives at least some of the credit for this transformation to the norm against state conquest first established in the Covenant of the League of Nations. The norm’s proclamation and subsequent enforcement by one of the world’s two hegemons (the United States) after 1945 has helped make territorial conquest taboo.

Hathaway and Shapiro also credit international norms for a decline in wars of conquest. They trace the transformation not to the League of Nations, but to the 1928 Treaty for Renunciation of War as an Instrument of National Policy. Often called the Kellogg-Briand Pact, after signers US Secretary of State Frank Kellogg and French Foreign Minister Aristide Briand, the treaty’s text is so short I’m going to quote it in its entirety:

ARTICLE I

The High Contracting Parties solemnly declare in the names of their respective peoples that they condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another.

ARTICLE II

The High Contracting Parties agree that the settlement or solution of all disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them, shall never be sought except by pacific means.

The pact, Hathaway told me — all 78 words of it — “was the first time the world outlawed war.” It marked an explicit repudiation of the old model of Grotius and others justifying wars of conquest. In outlawing war, it more importantly denied countries the benefits of war, like new territory. Spoils — at least of territory — could not be kept. “They not only prohibited the use of force but they took away the legal consequences or benefits of going to war,” Shapiro says.