They didn't. The US signed a ceasefire with North Vietnam ending their role in the conflict. Two years later the Northern forces defeated the ARVN, captured Saigon and reunited Vietnam.
President Nixon accomplised ending the Vietnam War.
nixon
Nixon, 1974
Lyndon B. Johnson
ending the Vietnam War
When President Ford ordered the US Embassy in Saigon evacuated in April 1975.
Richard Nixon
There is NO North Vietnam & South Vietnam today (and there hasn't been a north & south for over 30 years; when the north conquered the south, ending the war in 1975). Today's Communist Vietnam is President Triet.
He was in 1956 Eisenhower's Vice-President., When he was elected there was high inflation and economic recession from high spending in the war. His greatest success was easing coldwar tensions and with forign countries. He was impeached because of the Watergate Scandal but resigned before he was removed from office., 37th President of the United States (1969-1974) and the only president to resign the office. He initially escalated the Vietnam War, overseeing secret bombing campaigns, but soon withdrew 540,000 American troops and successfully negotiated a ceasefire with North Vietnam, effectively ending American involvement in the war. He was responsible for the Nixon Doctrine. He was also the first President to ever resign, due to the Watergate scandal.
Nixon became President because he wanted to run, his political party supported him, and he won the election (with ending the Viet Nam conflict as part of his platform).