Why did American policy makers believe that containing communist expansion should be the foundation of American foreign policy?

American policymakers believed that containing communist expansion was essential to prevent the spread of Soviet influence and the ideological battle between capitalism and communism during the Cold War. They feared that if one nation fell to communism, neighboring countries would follow in a "domino effect." This belief was rooted in the experiences of World War II and the subsequent rise of communist movements, leading to a commitment to support allies and intervene in conflicts where communism threatened to take hold. Ultimately, containment aimed to protect American interests and promote stability in regions critical to global security.