AskHistorians

The problem wasn't that the idea was bad, but that there wasn't an international order around to enforce norms.

World War I had several succession wars as empires fell apart and maps were being redrawn. Even with the end of those wars, there wasn't anyone willing to step in if a nation violated these norms. France backed down after invading Germany for reparations coal in the 20's, but Japan invaded China and wasn't going to be stopped by a toothless committee.

In contrast, the UN became important in the Cold War world order as it gave a place for the two great powers to talk and try to gain international acceptance for their actions. There was also a vested interest by nations to enforce international norms; you generally needed a veto by one of the top 5 in the Security Council if you were going to break something. Great Power conflicts still happened, but there was a greater chance that a smaller war would be intervened in.

The League Of Nations and the UN briefly existed at the same time in 1945-1946.

The League Of Nations had primarily been a European organization which the US never joined. It had also been ineffective at doing much of anything about conflicts in the 1930s, and during WW2 it continued to exist but was basically just in stasis. The UN was conceptualized during WW2 as a beefed up version of the League to accomplish the same general goal of international stability for a post-WW2 world. The UN included the US and USSR as permanent members, the two global superpowers. In the changed dynamic of the post-WW2 world, that gave it a much more global sort of reach than the League.

While the League Of Nations squabbled uselessly and failed to prevent wars and atrocities the UN on the other hand