In november 1967, general westmoreland told the american public that
the united states was making progress in vietnam
the vietcong were winning the war
the war would be a drawn out, inconclusive war
the end of the war was in sight

He said that they were making progress.  Though the Tet Offensive was a military victory, it was political victory for the North Vietnamese.  When scenes of the war were broadcasts to the American public, the reaction was so negative that support for the war dwindled.  This led to negotiations where the U.S. eventually withdrew from Vietnam allowing the North to conquer the South.