The U.S. intervention in Vietnam was influenced by the belief in the "domino theory." This theory suggested that if one country in Southeast Asia fell to communism, then neighboring countries would also follow suit, like a row of falling dominoes.
1. The domino theory was a key factor in shaping U.S. foreign policy during the Cold War.
2. It was used to justify the U.S. involvement in conflicts such as the Vietnam War.
3. The fear of communism spreading throughout the region was a driving force behind this intervention.
Therefore, the correct answer to the question is the "domino theory."
