President Richard Nixon, elected in November 1968, sought an exit strategy that would leave US credibility intact.
In June 1969 he announced a policy of "Vietnamization" – training and equipping the South Vietnamese military to enable the US to reduce troop numbers.
Over the following three years, more than 500,000 soldiers were withdrawn. This reduced already-low morale among troops, feeding high levels of desertion and drug abuse.
US public hostility continued, fuelled by several events. Two offensives against communist bases and supply routes in Cambodia, in 1970 and 1972, sparked waves of protest.
The June 1969 battle of "Hamburger Hill" raised fresh concerns about wasted US lives - 46 soldiers died fighting a successful but bloody battle for a site from which their comrades were withdrawn soon afterwards.
And evidence came to light of a 1968 massacre at My Lai, where US forces slaughtered more than 300 Vietnamese villagers during an assault on suspected Vietcong camps.
Ho Chi Minh died in 1969, but his successor Le Duan continued to fight. The communists launched another major offensive in 1972, but were turned back by massive US airpower.
Slow and convoluted talks were held in Paris from 1969. Punctuated in 1972 by an intense eight-day US bombing campaign targeting Hanoi, the negotiations eventually produced a peace deal in January 1973.
Under the agreement, US forces would leave and South Vietnam would have the right to determine its own future.
The last American troops left in March 1973, but what some described as a "post-war war" continued.
Southern and northern forces accused each other of breaking the terms of the truce and fighting continued, although it was less intense and casualties were lower than in previous years.
American aid to south Vietnam decreased and the southern government became progressively weaker.
In early March 1975, buoyed by a successful operation at the start of the year, Hanoi launched the first phase of an offensive to take the whole country.
The south Vietnamese army crumbled faster than expected, and in seven weeks communist forces had swept through the south taking the central highlands and the east coast. Millions of refugees fled towards Saigon.
On 21 April, with the NVA closing in on Saigon, the South Vietnamese President Nguyen Van Thieu resigned and fled to Taiwan, railing tearfully against the US.
Six days later the city was surrounded. The NVA began launching shells into civilian areas. Looting broke out.
On 29 April the US ordered the helicopter evacuation of 7,000 American administrators and South Vietnamese from the city. Refugees battled to join the exodus.
The following day, NVA tanks drove unopposed into central Saigon. The city was renamed Ho Chi Minh City.
In the wake of the NVA victory, hundreds of thousands of south Vietnamese who had opposed the communists fled by boat, fearing reprisals.
They formed the first wave of Vietnamese boat people – others followed in 1978 fleeing communist economic reforms.