American arrived in Vietnam as early as 1948 under President Truman but they were mostly there to provide support and advisement. The government of South Vietnam proved to be corrupt and incapable causing the US to increase their presence
Every war has a starting point and the starting point for Vietnam was the Gulf of Tonkin incident. Sailors of the USS Maddox and the USS Turner Joy reported being attacked by the North Vietnamese. As a result, President Johnson asked Congress for full authority to wage war in Vietnam - called the Gulf of Tonkin Resolution.
American troops in Vietnam increased steadily until 1968. That year the country saw:
The Tet Offensive
The My Lai Massacre
The assassinations of Martin Luther King and Robert Kennedy
President Johnson leaving the White House
By 1968, Americans were tired of the War in Vietnam.