Escalating war (1965 - 67)
In February 1965, with the South Vietnamese army (ARVN) making little headway against the Vietcong, the US launched Operation Rolling Thunder, a sustained bombing campaign against targets in north Vietnam.
But it quickly became clear that US airpower alone was doing little to halt Vietcong operations in the south.
In July 1965 the US announced a deployment of 100,000 US soldiers. Australia, New Zealand, Korea, the Philippines and Thailand also contributed troops.
The strategy was one of gradual escalation and attrition, aimed at destroying the communists' capacity to fight, rather than taking territory.
The next two years saw major battles near Danang and Ia Drang, and large-scale US operations against communist bases.
But despite sustaining heavy losses, the communists fought on and returned to re-occupy areas after US forces had left.
Vietcong guerrillas were often difficult to distinguish from civilians and moved effectively in the difficult terrain.
US aircraft sprayed millions of gallons of the toxic herbicide Agent Orange over the jungle to destroy the thick foliage that Vietcong fighters used for cover.
Heavy bombing, including the use of napalm, continued. But the communist forces fought back with anti-aircraft guns and fighter jets supplied by their Soviet and Chinese allies.
By the end of 1967, there were 485,000 US soldiers in Vietnam. US troop losses and civilian casualty figures were triggering domestic protests, and the outcome of the war remained uncertain.