A few lesser known details of the Tet Offensive.
At the head of the list, the Viet Cong really had no choice to carrying out the Tet Offensive, first because the at-the-time almost unknown Phoenix (counterinsurgency) Program, was devastating them, tearing them apart. They had only weeks or months left before they were annihilated.
The second reason was that they were ordered to carry out the Tet Offensive by the North Vietnamese Army, who had lost patience with the Viet Cong and wanted them to either win by overthrowing the southern government, or to die trying.
The bottom line is that despite their best efforts, after the Tet Offensive, the Viet Cong were effectively wiped out. While indeed caught off guard, the Americans and the South Vietnamese ARVN responded forcefully and effectively and destroyed the Viet Cong utterly.
And the Communists erroneously thought that the Tet Offensive would trigger a general uprising of the South Vietnamese people who would rise up and throw out the Americans. They thought they would win and this would be their final offensive.
That is why they murdered with impunity over 3 thousand South Vietnamese civilians in Hue and buried them in mass graves north of town. Of course, no uprising happened and the local VC forces were decimated by our guys. But the mass murders at Hue were a sign of things to come later when the communists finally got control in 1975