Answer:
A. domino theory
Explanation:
Vietnam was a communist stronghold in Southeast Asia, having close ties with the Soviet Union. The United States were not very fond of it, and apart from having Vietnam, a country with excellent strategic location, becoming communist, the real fear was that it will be a domino effect. The US administration was convinced that if Vietnam becomes communist, than the trend will spread in the region of Southeast Asia, thus making a very large region with enormous potential as enemy, while being ally with the Soviets. Because of this fear, the US attacked Vietnam, only to be ashamed by being defeated.
