> The League of Nations existed, but was an abject failure for a number of reasons and failed to prevent WWII.

UN did not prevent WWIII. It is the mere fact that nuclear weapons and MAD made the global superpowers realize that there is no winning in such war. And that a security competition and proxy wars are acceptable. But direct confrontation to be be avoided at all costs. It is not like US, Russia (probably china and others too) do respect the UN that much anyway.

If nuclear weapons existed before WWII, There is a slim chance that it would happen.

UN did not protect iraq and Ukraine from illegal invasions such that League of nations did not protect Ethiopia in the past. The mere fact that in all cases no body wanted to confront a major power invading another country far away.