The domino theory was based on the assumption that the countries of Southeast Asia were all relatively weak and vulnerable to communist takeover. If one of these countries fell to communism, it was argued, the others would soon be taken over as well. This would create a communist bloc in Southeast Asia that would be a major threat to the United States and its allies.
The United States intervened in Vietnam in order to prevent the country from falling to communism and, potentially, setting off a domino effect that would lead to the spread of communism throughout Southeast Asia. The United States also intervened in order to protect its own interests in Vietnam, including access to oil and other natural resources.
The domino theory was criticized by many people, who argued that it was based on an inaccurate assessment of the political and economic situation in Southeast Asia. It was also argued that the United States intervention in Vietnam only made the situation worse and did not prevent the spread of communism.