That was the time after WW2. Of cause the economy does a lot better, when you do not bomb out cities and kill millions. That could have happened earlier, without the war and everybody would have been better of.

The US is at war and China is not. The third most powerful country would be India and afaik they are not at war right now.

WW2 was not a boon to production. Well it was for weapons, but most of Europe and good parts of Asia were destroyed with millions dead. The only country coming out of it in better shape was the US, which did not see bombing and had relatively low human losses(still horrible to be clear, but not even close to what others had).

Modern wars suck and any smart country will try to avoid them. Hence you see Russia being fucked over by the EU via Ukraine and China fucking the US via Iran. Obviously you need dumb leaders to walk into those traps into the first place.