The incident started in the beginning of 20th century when a Vietnamese nationalist Ho Chin Minh was studying in U.S and wrote to president to seek their help to run the country which was then under French but all this was not attended by U.S that time. Afterwards they approached this same man and said that after their help to fight against Japan U.S will definitely help Vietnamese to get their independence. But U.S moved from its word then people in Vietnam united under Minh to stand against France. At that time a treaty named S.E.A.T.O. was signed by Vietnam. North was the communist and south became democratic.
Truman and western allies took communism as the big threat after World War II in December 1949 Chinese communist forces met victory in the civil war. Now France tried to capture the control over south East Asian countries including Vietnam in 1945 after the world war ended. U.S feared that the communism may spread to the other countries. U.S felt that they should now fulfill the needs of Vietnam to avoid it going into the hands of Ho Chi Minh which will again unite the Vietnam and form communist rule. So they started supplying weapons to south to help them so that they can protect themselves from rebels. U.S sent combat troops to South Vietnam. So the two groups started clashing.
War started in 1959 and was one of the brutal battles fought between Democratic Republic of Vietnam and Republic of Vietnam. It was ended after 16 long years. Many men lost their lives. Finally a treaty was signed in which U.S army has to move out of Vietnam and it was united again.