What does the domino theory refer to regarding communism?

The concept of the domino theory suggests that the spread of communism in one country could lead to a chain reaction, where neighboring countries would also fall to communism. This theory was especially prominent during the Cold War, as it shaped U.S. foreign policy, particularly in Southeast Asia. The underlying concern was that if a country like Vietnam were to turn communist, it might inspire or catalyze similar movements in adjacent countries, threatening regional stability and U.S. interests.

This idea played a significant role in justifying interventions, such as in Vietnam, as U.S. leaders believed that preventing communism in one nation was crucial to safeguarding neighboring nations from the same fate. The other options do not align with the essential premise of the domino theory; therefore, option A captures the essence of the theory accurately.