> America became the richest, most powerful country
More accurately, this happened because every other power was laid to waste by two successive world wars. You'd have been hard pressed to find a standing factory in Germany, Britain, or Russia in June of 1945.
Further, the US has had several boom and bust cycles people only really hear about 'the great depression' but there have been plenty of major slumps in the US economy for example around the Civil War. So, a nominal lead in good times is not that meaningful.
So it was only during WWII that the US went from near parity to a massive and sustained lead. And it did not take that long before we gave that up to either China or the EU depending on how you want to slice it.
Uh the US is a far bigger place? The civil war kicked off our version of the second industrial revolution, and the frontier (see ailroads) acted as starter fluid.
Obviously US was looking great at that time, hence the immigration boom. (As Mark Twain went for in the name, the guilded age did have plenty of issues, but so did Europe. Class was strictly more palpable there, generally speaking.)
Then, as others with that firm foundation, we surfed tsunami of two world wars that swallowed everyone else. That seal Ed the deal (and the second got us out of the great depression).
I'm being a bit hyperbolic, but I think my point stands that the US had many more factories left after the war, not to mention working age men. The US also had the Marshall Plan to repurpose that capacity into making products for domestic and foreign consumption.
