The correct answer to this open question is the following.
The reason for the United States of America's departure from Vietnam in 1973 was that President's Nixon administration was already under heavy scrutiny and criticism for maintaining the United States troops installed in South Vietnam.
After very difficult moments such as teh Tet Offensive, the American society in general, and the young Americans in particular, started to seriously question the participation of the United States Army in the Vietnam War, concluding that was not a US war. They started to demand the withdrawal of the US troops from Vietnam and organized many protests and marches to demand action on the part of the federal government.
Finally, in January 1973, a peace agreement was signed and the US started to withdraw its troops. The process lasted until March 29, 1973.
However, the United States contib¿nued to support
South Vietnam, applying the foreign policy of containment to impede the spread of Communist over South Vietnam.
