Final answer:
The domino theory was a political theory during the Cold War that suggested the spread of communism from one country to another, influencing US policy and military intervention in Vietnam.
Step-by-step explanation:
The domino theory was a political theory during the Cold War era that suggested that if one country in a region came under communist control, neighboring countries would follow suit, like dominos falling in succession. This theory was influential in shaping US policy, particularly in Southeast Asia.
The US believed that if Vietnam fell to communism, it would lead to the spread of communism to other countries in the region. The domino theory influenced US involvement and military intervention in Vietnam.