Answer:
The Vietnamese were not expected to be able to mount such a large offensive.
Step-by-step explanation:
The Tet Offensive, officially called "The General Offensive and Uprising of Tet Mau Than 1968" in Vietnam, was a large-scale military campaign during the Vietnam War. Fought between 30 January - 23 September 1968, it was launched by the North Vietnamese People's Army of Vietnam and the National Liberation Front of Vietnam (the Viet Cong), against positions held in South Vietnam by the Army of the Republic of Vietnam and the United States armed forces. Its name comes from the Tet holiday, the Vietnamese New Year, which was the date when the offensive started.
By early 1968, the United States military leaders were convinced that the North Vietnamese were not capable of mounting any significant large operation and were even on the verge of defeat. General William C. Westmoreland, commander of the US armed forces in Vietnam, declared on 21 November 1967 that the Vietnamese communists were "unable to mount a major offensive. I am absolutely certain that whereas in 1965 the enemy was winning, today he is certainly losing. We have reached an important point when the end begins to come into view". However, in a completely unexpected move, the Vietnamese were able to movilize almost 600,00 regular troops and guerrilla fighters in a wide front that encompassed almost all of South Vietnam. While the communists led failed on their attempt to trigger a popular uprising in South Vietnam, the magnitude of the offensive and the prospects of a war that could stretch on for many years to come came as a terrible shock to public opinion on the US. Support for the war diminished as protests against it raged on, and the US started to look for an exit to the war. The Vietnam War ended in 1975 when communist Vietnamese troops captured the southern capital of Saigon, and Vietnam was reunified as the Socialist Republic of Vietnam in 1976.