During the Cold War, US foreign policy changed. Prior to World War II, America generally had an "isolationist" policy whereby the US avoided getting entangled
in foreign conflicts or the internal affairs of foreign governments (although there were exceptions to that policy regarding Hawaii, Cuba, Puerto Rico, the
Philippines, Panama, Mexico, etc.). But during and after World War II the US had permanently cast aside its isolationism in favor of a policy of
"containment," specifically containing the spread of Communism wherever it appeared around the world.
Why did the US government consider it necessary to stop the spread of Communism?