Fifteen nations signed the Peace Pact on [ 27 August 1928] and within a year nearly every state in the world followed suit. For the first time in the history of the world, war was illegal.

More on https://www.theguardian.com/news/2017/sep/14/making-war-illegal-changed-the-world-but-its-becoming-too-easy-to-break-the-law