After WWII, America could have set up a one world nation government back then, and immediately after the war, put whomever they wanted in charge of what used to be other countries, but that would have become more like states are in the US right now, and could have even redefined borders, or created multiple different states out of other, especially more larger nations/countries if they wanted to, or needed to, or done all kinds of things reallly, etc, pretty much anything they wanted really, etc, and there would have never been a such thing as other nations or countries getting nuclear weapons, because no matter where they had them or put them in the world, they would have always been in the sole hands and control of the new overarching one world government, which would be or would have been much like the federal government in the US is today with it's states, etc... and so on and so forth, etc, etc...
And if they set it up so that these new "states" got to keep their own more local cultures and cultural identities, and make their own laws and rules and policies kind of like the states can and do in the US today, but that could not violate the rules and laws of the new overarching "federal-like" government, I think they would have had relative peace for awhile, and maybe a long while afterwards, etc...
But, we'll never know, because that didn't happen... And I'm not necessarily saying it should have happened either, unless time proves me wrong right now of course, etc, which it might, etc, we just don't know right now or yet right now, etc, if it was truly the right decision back then or not right now, etc...
There would already be a kind of democracy everywhere by now though, etc, and would have been immediately, right after the war, etc, and countries like China and Russia would not be anything like they are right now today, or how they became right up to now and after the war, but would have democracies already, and would be more like states are in the US right now, etc...
Would it have been better...?
I God's honestly, or honest to Gods-ly, "don't know"...? But I think what happens in the, possibly immediate, or maybe relatively immediate, future, will probably come to answer that for us right now, etc...
God Bless!