The U.S. became involved in Vietnam’s conflict because a. it feared the Tet offensive. b. Viet Cong forces attacked U.S. troops. c. it feared the domino effect. d. it had a mutual-defense agreement with South Vietnam.
The U.S. became involved in Vietnam’s conflict in part because "c. it feared the domino effect," since this held that as soon as one country feel to communist rule, communism would spread throughout the world.