>>513144168
The US has a history of oscillating between isolationism and internationalism.
Whenever they went isolationist, the world turned to shit, and they had to bail out Europe (twice). Then they go all out being internationalists, and end up in forever wars, so they scale back, and look inwards.
The reason they teched so hard into military, is basically because they had the money. A richer country attracts talent, so over time there has been a brain drain, with the best minds going to America, where their efforts are rewarded, more so than in their own countries.
But mainly they just had the industrial capacity and an economy that scaled really well. They weren't technologically ahead in WW2 (the Sherman was objectively shit), but getting Nazi scientists helped their air force and rocket program get a real boost.
Once they saw the potential the jet engine had, together with their industry and economy winning out massively over the rest of the world in WW2, naturally they would end up with the most insane army the world has ever seen.