Final answer:
The Domino Theory was an important factor in U.S. involvement in Vietnam as it was a belief that if one country fell to communism, neighboring countries would also fall. This theory led to increased U.S. military intervention in Vietnam to prevent the spread of communism.
Step-by-step explanation:
The Domino Theory was a belief held by the United States during the Cold War that if one country in a region came under the influence of communism, then neighboring countries would also follow in a domino effect. The theory played a significant role in U.S. involvement in Vietnam because the U.S. feared that if Vietnam fell to communism, other countries in Southeast Asia would also fall. This fear led to increased U.S. military intervention in Vietnam in an attempt to prevent the spread of communism.