I've already formed a small answer, WW2. However, I'd like to know the exact reasons and causes, AND WW2 might not even have anything to do with it. Today, the US are widely known as the 'world police' whereas pre-WW2 they hardly ever paid attention to anything outside North America. What caused this?
The main cause for America's transition from isolationism is contained in NSC-68, a National Security Council document written for Truman that described the global threats facing the US. Released on April 7, 1950, its primary authors, Paul Nitze, Dean Acheson, and Chip Bohlen declared that to protect the free world from communist aggression, the US must abandon any plans for slipping back into isolationism. It recommended a substantial increase in defense spending that included growing the military's strength and investments in missile delivery systems for nuclear weapons. Two months after the document's release the Korean War broke out, lending credence to the writers' predictions about communism.