At first, after the Geneva Accords split Vietnam in two (South and North Vietnam), a group called the Vietcong began a war against the governing authority in South Vietnam. During this time, the US was sending supplies and a small amount of troops to South Vietnam to help shape the new democratic country. When North Vietnamese (communist) patrol boats attacked American destroyers, this led to all out war with North Vietnam. The US also got involved to stop communism from spreading from North to South Vietnam.
