What was the main reason for the United States' involvement in the Vietnam War?

To prevent the spread of communism
To gain territory in Asia
To spread communism
To support Vietnam's independence

The main reason for the United States' involvement in the Vietnam War was to prevent the spread of communism. The US government was concerned about the domino theory, which suggested that if one country in Southeast Asia fell to communism, neighboring countries would also fall in a chain reaction. This belief led to the US providing military and financial support to South Vietnam in order to prevent the spread of communism in the region.