Tet is the holiday that celebrates the Vietnamese lunar New Year and during the war the event was usually marked by a cease-fire to allow the South Vietnamese to enjoy their celebrations and observe their religion.  However, intelligence information collected in the early weeks of 1968 indicated that the Viet Cong, with assistance from NVA units in South Vietnam, was going to mount a large scale offensive.  The attack was not expected until February so when the Tet Offensive opened on 30/31 January it was unexpected not only in its severity but also in its timing.  It was estimated that around 80,000 communist troops were involved in the offensive and that as many as 10,000 of these were killed or captured.  However, while the offensive might have been a military defeat for the VC and North Vietnam, it was certainly a huge political and psychological victory for the communist forces.  Until Tet the American public had been told that the communist threat was being blunted in South Vietnam and that the North was about to buckle under the pressure of the Rolling Thunder campaign.  The Tet Offensive made it clear that the communist threat in South Vietnam was still very real and that the 486,000 Americans based in that country were unable to destroy the VC.  Coupled with increasingly heavy aircraft losses over North Vietnam, 1968 was probably the lowest point of the war in Southeast Asia with the exception of the final defeat in 1975.

The Tet Offensive opened on the night of 30/31 January with simultaneous attacks on a number of South Vietnamese cities and towns as well as 23 US and Vietnamese air bases.  However, some VC units in the northern and central provinces of South Vietnam moved prematurely and attacked seven bases, including Da Nang and Pleiku, in the early hours of the 30th.  These attacks further warned the Americans and South Vietnamese of the imminent major offensive.  Within hours of the main offensive starting the American Embassy in Saigon was occupied by enemy troops and the old Vietnamese imperial capital at Hué was captured.  The success of the airfield attacks encouraged the Viet Cong and North Vietnamese Army as the incidence of such attacks increased after Tet.