Turning point (1968)
On 31 January 1968 the North Vietnamese Army (NVA) and Vietcong launched a large-scale offensive during the Vietnamese holiday of Tet.
In surprise simultaneous assaults they attacked 36 provincial capitals and five of the six major cities, including Saigon where they penetrated the US embassy compound.
Ten days earlier they had begun an assault on a US marine base near the DMZ, successfully diverting US attention from the forces massing elsewhere.
Although caught off guard, US and ARVN repelled the onslaught relatively successfully.
The communist forces failed to hold any sites for more than a few days - apart from the city of Hue, which they held for three weeks. The Vietcong particularly suffered heavy losses.
But the offensive hit US public opinion hard.
Graphic film footage of the fighting reinforced concerns about casualties, and the fact the offensive took place undermined the White House's claims that victory had been in sight.
Political support for President Johnson waned. In March he halted bombing, called for peace talks and said he would not run for a second term in elections in November 1968.
Heavy fighting continued on the ground, however, as both sides struggled to capitalise on headway they believed they had made.