Question

What was the domino theory?

the opinion that war is just a game played between nations, but that it becomes horribly real to those taking part in it

the view that fighting in Southeast Asia was inevitable for the United States because the war was destined to happen

the belief that domination of Southeast Asia was a critical factor for the Western world to expand socialism

the theory that if South Vietnam became Communist, Communism would spread throughout the rest of Southeast Asia

Answer 1

Answer: last one

Explanation: the theory that a political event in one country will cause similar events in neighboring countries, like a falling domino causing an entire row of upended dominoes to fall.