Why did the Vietnamese Tet Offensive mark a dramatic turning point in the war?

The Tet offensive marked the moment when what had been seen as a manageable overseas conflict turned into something much more messy. The assaults launched by the Viet Cong against the South Vietnamese and their US Military allies in early 1968 were a military fiasco. But the size of the offensive, the dramatic increase in US troop numbers it triggered, and the consequent loss of life on all sides, weighed heavily on US public opinion. The offensive also involved a draining battle for the Khe Sanh airstrip, used as a US Marine base. Khe Sanh became a symbol of the war's futility, abandoned as it was in June 1968, deemed to be of no strategic worth