"The Internationalists" makes the case that the 1928 Kellogg-Briand Peace Pact to outlaw war was what started the paradigm shift against war [1]. (Of course the thesis is controversial, as you say.) You might enjoy the book; here's an Op-Ed the authors wrote in summary [2], and a podcast interview with the authors [3].

[1] https://www.goodreads.com/book/show/30753784-the-internation...

[2] https://www.nytimes.com/2017/09/02/opinion/sunday/outlawing-...

[3] https://www.lawfareblog.com/lawfare-podcast-oona-hathaway-an...

> From 1816 until the Kellogg-Briand Pact was first signed in 1928, there was, on average, approximately one territorial conquest every 10 months. Put another way, the average state during this period had a 1.33 percent chance of being the victim of conquest in any given year... The average amount of territory seized between 1816 and 1928 was 114,088 square miles per year...

> Since World War II, conquest has almost come to a full stop. The average number of conquests per year fell drastically — to 0.26 per year, or one every four years. The average size of the territory taken declined to a mere 5,772 square miles per year. And the likelihood that any individual state would suffer a conquest in an average year plummeted — from 1.33 percent to 0.17 percent, or once or twice a millennium.