Previously, the United States had a strict isolationist attitude. They did not want to get involved in any foreign conflicts they didn't have to.
However, as the communist Soviet Union rose, and the rush from the U.S. to spread democracy and stop communism started, America turned away from its isolationist attitude and started getting their hands in all different places.
The Truman Doctrine aimed to aid nations (especially those war torn by WWII) where the spread of communism was imminent.
