The major change in U.S. foreign policy following World War II was the shift towards containment of communism, shaped by the emergence of the Cold War. The United States adopted the policy to prevent the spread of communism globally, particularly in Europe and Asia. This led to various strategies, including economic aid through the Marshall Plan to rebuild Western Europe, military alliances such as NATO, and interventions in conflicts like the Korean War and later, Vietnam. The containment strategy became a cornerstone of U.S. foreign relations, influencing American engagement worldwide until the collapse of the Soviet Union in 1991.