Question

ow did the domino theory influence the United States' role in the Vietnam War?

Answer 1

The domino theory meant that if Russians had brought Communism to Vietnam, it's neighbouring countries would also be influenced (Malaysia, Indonesia, Korea, China, etc.)
The United States (Capitalists) saw the spread of communism as a threat to their ideology. Spread of Communism meant russia would grow stronger and stronger and the US would be weaker (relates to the Cold War between the two)
So later on North Vietnam and South Vietnam was used by Russia and the U.S respectively and a huge war broke out between Vietnam and US (as they became too heavily involved)