Tet was a largely a change in public perception. Until this point the American public was being told that America was "winning" the conflict. They had seen the news footage of our huge air campaigns as well as reports on body counts and were being told that the NVA/VC could not sustain the conflict at this rate forever. In many ways this was similar to Bien Dien Phu as in a single event the public began to question our end game. The imagery of insurgents breaching the wall and attacking the US embassy in Ho Chi Mihn City led the public to begin to question our ability to win the conflict.

Just a general reminder about homework questions here. If the OP is upfront about it, we aren't going to remove the post. However, we aren't here to do anyone's assignment. A good answer to these types of threads is one that provides some good sources, and helps the OP find the information themselves!