During WWII, they called themselves the "Arsenal of Democracy", meaning that they were the democratic nation that could deal out all of Democracy's justice for the rest of the world (I know, right). Also, the US had conflicts with the Soviet Union, which was the lead communist country. Really, the US was the only one that could match the Soviet Unions power. While the US was the first to build the atomic bomb, Soviet Union soon caught up and built one five years later. Same thing happened with nuclear missiles. The space race was first started by the USSR when they sent Sputnik into space. Therefore, through everlasting conflict with the Soviet Union as well as the fact that the US had many thousands of nukes, America thought it right to be globally responsible for communism
Hope this helped! Sorry it was a bit long :D
