The answer is: b. to prevent the spread of communism in Southeast Asia
After the Geneva Accords divided Vietnam into South and North Vietnam, a group called the Vietcong started a war against the South Vietnam government. Meanwhile, America was sending supplies and a small amount of troops to South Vietnam to support the new democratic country. However, when the communist North Vietnamese defending boats attacked American destroyers, it caused a war with North Vietnam. The US also got involved to stop communism from expanding from North to South Vietnam.
