Why was the domino theory a reason for the US to get involved in the Vietnam war?

The domino theory posited that if one country in Southeast Asia fell to communism, neighboring countries would follow, like a row of falling dominoes. U.S. policymakers believed that a communist victory in Vietnam would threaten other nations in the region, potentially leading to a broader spread of communism. This fear drove the U.S. to intervene militarily in Vietnam, aiming to contain the spread of communism and maintain geopolitical stability. Ultimately, the theory underscored the U.S. commitment to its Cold War policy of containment.