Answer: The Tet Offensive.
The Tet Offensive (officially The General Offensive and Uprising of Tet Mau Than 1968) was one of the largest military campaigns of the Vietnam War, launched on January 30, 1968 by forces of the Viet Cong and North Vietnamese People's Army of the Republic of Vietnam against the forces of the South Vietnamese Army of the Republic of Vietnam, the United States Armed Forces, and their allies.
The offensive shocked the U. S. public and had a strong effect on the U. S. government. Americans had thought that the North Vietnamese were being defeated and were incapable of launching such an ambitious attack. Public support for the war significantly decreased, the media became more critical and the U.S. sought negotiations to end the war.