A couple of events come to mind, part of the winning side in WW1 and WW2
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The US became an industrial giant equipped with nuclear power.
The United States was the nation that emerged from World War 1 as the greatest world power. By remaining neutral for most of the war and the land-lease act the United States was able to leave the war with more power than before.
the end of the Cold War.
The scope of its power increased.
Isolationism