The transformation of American foreign policy post-World War II was primarily driven by the emergence of the Cold War. The U.S. sought to contain the spread of communism, largely associated with the Soviet Union, leading to a strategic pivot from isolationism to interventionism. This involved military alliances such as NATO, economic initiatives like the Marshall Plan, and involvement in conflicts such as Korea and Vietnam. The policy of containment influenced American actions and ideologies for decades, establishing a framework for international relations that prioritized democratic governance and capitalist economies, shaping global dynamics in the latter half of the 20th century.