Basically freedom is one of the founding principles of the US governmetn. Hope that helps
I believe it's from non-interventionist to interventionist.
During the cold war period, the Soviet Union tried to spread communism influences throughout the world by giving supplies and weapons to conflicted nations in order to gain an ally.
Because of this, united states believe that they need to start making intervention for global conflicts to prevent communism from gaining dominant control in the world affairs.
