What was the name given to the idea that if vietnam fell to communism, the rest of southeast asia would become communist? a. the domino theory b. the steamroller theory c. the roll back theory d. the communist theory

Hello!

This was known as A. The Domino Theory.
The United States made an effort to prevent Vietnam from falling to communism. If it did, it was believed that Vietnam's neighboring countries would also fall to communism afterwards. The term "domino theory" relates to dominoes toppling over and causing dominoes around them to fall as well. 

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