Question

Which of the following best describes the “domino theory,” which was used as a justification for US involvement in Vietnam?
If the US can defeat one Communist country, it can defeat all of them.
If one section of a country is taken over, the rest of the country will also fall.
If an aggressor nation invades one country successfully, it will invade another.
If a country falls to Communism, then its neighbors are also likely to do so.

Answer 1

"If a country falls to Communism, then its neighbors are also likely to do so."
The domino theory held that one nation's communist takeover would lead to a chain reaction of neighboring countries falling.