In 1975 with the defeat of the South by the North.
Vietnam had been at war for decades. There was a Vietnamese resistance movement before World War 2 fighting French colonial occupation. The during World War 2 they had to fight the occupation of Japan.
When World War 2 ended, The French assumed they would simply regain colonial control. However the Vietnamese resisted and comprehensively defeated the French at Dien Bien Phu.
At Geneva a Peace Agreement was drawn up which divided Vietnam into communist controlled north and American backed south.
The President in the south Diem, was supposed to hold elections but didn’t. His oppressive regime led to greater opposition and by the early 60’s a civil war was developing. The Americans removed Diem and his brother through a military coup. However this was no more successful in stopping communist gains.
The Americans, particularly under Johnson escalated involvement to over 500,000 troops but could not secure victory.
Johnson was succeeded by Nixon in 68. His Vietnamization policy aimed at allowing the US to withdraw with the confidence that the South Vietnamese government and forces could survive.
The Americans pulled out in 73 and within 2 years South Vietnam, along with Cambodia and Laos, fell to the communists.