[jdietrich]

America had one overwhelming advantage - thousands of miles of ocean separating it from enemy lines. The industrial capacity of practically every other major nation was laid waste by strategic bombing. America also had domestic production of nearly every important manufacturing input, rendering it practically immune to naval blockade.

Post-war, America didn't spend a single cent on rebuilding infrastructure lost to bombing, while the other major powers needed a decade of austerity just to get back to where they started. Britain was still repaying the Anglo-American war loan in 2006.

America did an excellent job of scaling up manufacturing capacity during WWII, but the factors that made it such a dominant force were largely accidents of geography.

[DrScump]

  America didn't spend a single cent on rebuilding infrastructure lost to bombing

... within the continental U.S.

America spent a lot in helping rebuild infrastructure elsewhere.

[kahnpro]

Incidentally this is also a major overlooked factor in the Cold War against the USSR.

America has had 150 years of peace and stability. The Soviet Union was ravaged by a traumatizing war at home and before that a major civil war and revolution.