Final answer:
The domino theory was part of the Cold War containment policy that suggested if one country fell to communism, the surrounding ones would likely follow, shaping U.S. involvement in places like Vietnam.
Step-by-step explanation:
The domino theory was an element of U.S. foreign policy during the Cold War outlining the belief that if one country fell to communism, neighboring countries would also fall, similarly to a row of dominos toppling over. This theory was a component of the larger containment policy, which aimed to restrict the spread of communism and the influence of the Soviet Union. The domino theory found its prominence under President Dwight D. Eisenhower and shaped U.S. engagement in places such as Southeast Asia, particularly Vietnam, as policymakers believed that the fall of a single nation to communism would threaten the stability and political orientation of neighboring nations.