In the First World War (1914-1918), Germany fought France, Britain, Russia, and the United States. Germany lost–barely. The French sought to create a post-war peace system based on keeping Germany weak. Break up Germany into smaller states; grant the French control of the Rhineland and the Ruhr (Germany’s industrial heartland). The British and the Americans didn’t like this solution, which just promised future wars. Britain and the United States came up with a different plan: they would guarantee French security with an alliance treaty. If Germany (or Mars) attacked, Britain and France would come to the aid of France. However, the United States Senate refused to approve the Versailles Treaty (and its obligations for the United States). The British took the view—not entirely reasonable in light of the subsequent German danger under Mr. Hitler—that this let them off the hook as well. All of a sudden, the French had neither an American nor a British alliance, nor did they have a weakened Germany. What to do?
They tried coercing the Germans by occupying the Ruhr (1923-1925). Unfortunately, they owed American banks a ton of money from the war. So the American could—and did—bend France over the couch. This led to the Dawes Plan and, eventually, to the Locarno Agreements.
Aristide Briand (1862-1932) fell heir to this mess. Briand was a leftist politician who had been prime minister on many occasions. In 1925 he became foreign minister. He needed a way to fend off a future war with Germany. Partly, this meant sucking-up to Germany. Partly this meant trying to snare the United States into promising to defend France. Briand fished around, then proposed what amounted to a defensive alliance between the US and France.
Frank Kellogg (1856-1937) grew up in the Upper Mid-West, taught himself law under the old pre-law-school system, and eventually became a terrifying lawyer for the U.S. Government in the Theodore Roosevelt administration. He prosecuted the Union Pacific Railroad and Standard Oil. What he didn’t know about the real meaning of legal agreements wasn’t worth knowing. He became a Senator from Minnesota (1916-1922). Unlike most Republicans, he voted for the Versailles Treaty, so he lost that job. “Progressive” Republicans like Herbert Hoover didn’t hold it against him that he had stood up to the old men and idiots. He spent a year as Ambassador to Britain (1924-1925), then became Secretary of State (1925-1929).
So, Frank Kellogg had to deal with Aristide Briand’s proposal. How to dodge a French trap? He counter-proposed an agreement that would be open to every country and which rejected aggressive war as an instrument of national policy. Who could be against a rejection of aggressive war? In the public mind, the Kellogg-Briand Pact “outlawed war.” Cheering followed. Robert Ferrell told this story well in Peace in Their Time (1952)
Then Japan attacked China and Germany ran amok in Europe. The Second World War followed. The Holocaust followed. The atom-bombing of Japan followed. Filled with disgust over humankind, people came to misunderstand the Kellogg-Briand Pact. First, “nouveau realists” saw it as a joke. “Outlawing War” is joke, yes? More recently, lawyers have seen it as the entering wedge for the rule of law, norms, and a rules-based system. Neither is true. The Pact is best understood as a “realist” diplomatic maneuver in an age of popular idealism.
 This is a complex story.
 Yes, Republicans used to do this, just like the Democrats used to be an arm of the KKK.
 Oona Hathaway and Scott Shapiro, The Internationalists: How a Radical Plan to Outlaw War Remade the World (2017). The reviews aren’t much more sensible, even when written by historians