> They "failed" because they were gutted by a covert, powerful and violent "right-wing" alliance (imperialists, capitalists, white supremacists, all who felt tremendously victimized by recent global events incl. rise of communism). They were not about to throw away centuries of dominance to share international power with "inferior" classes of human.
It seems no answer can satisfy you.
You're saying we need more international organizations (or more powerful ones) to prevent the problems described. But when we do form these international organizations, and they don't do exactly like you hoped it's because of intangible reasons that cannot be falsified.
> The UN was rather effective in its initial decades.
The League of Nations, which precedes the UN, was rather effective in the 1920s and didn't include the USA. It also completely dropped the ball by 1930. Historians partially blame the League of Nations for the outbreak of second World War.
> Do you really think Hammarskjold, Kennedies, African(-American) leaders getting assassinated en masse in the 1960s was a spate of random coincidence? They were all united in opposing this covert alliance.
You're vacillating between wanting more powerful international organizations, but at the same time don't want international organizations wielding their power in ways you personally don't approve of.