Why didn't we listen to Von Neumann?

[Ezcheeze]

I didn't want to hijack the other thread as it's mostly discussion about whether he was right or wrong.

My question is why did the United States not try to basically take over the world? It seems obvious that had Germany or Japan been the first to create large amounts of nuclear bombs that they would have used them to take over the world. It's likely Russia would have done the same if they had been first. Why not the United States?

I suspect that if the first nuclear power had been a dictatorship, then it would have been very likely to make a move at controlling the world and supressing any other county's ability to produce nuclear weapons.

Could it be that Democracy saved the day (Or condemned the day if eventually theres a massive nuclear war that ends civilization)?