The Tet Offensive was a major escalation and one of the largest military campaigns of the Vietnam War. It was launched on January 30, 1968 by forces of the Viet Cong (VC) and North Vietnamese People's Army of Vietnam (PAVN). It was a campaign of surprise attacks against military and civilian command and control centers throughout South Vietnam. The purpose of the wide-scale offensive by the Hanoi Politburo was to trigger political instability, in a belief that mass armed assault on urban centers would trigger defections and rebellions.

The offensive was a military defeat for North Vietnam, as neither uprisings nor ARVN unit defections occurred in South Vietnam. However this offensive had far reaching consequences due to its effect on the views of the Vietnam War by the American public and the world broadly. General Westmoreland reported that defeating the PAVN/VC would require 200,000 more American soldiers and activation of the reserves, prompting even loyal supporters of the war to see that the current war strategy required re-evaluation. The offensive had a strong effect on the U.S. government and shocked the U.S. public, which had been led to believe by its political and military leaders that the North Vietnamese were being defeated and incapable of launching such an ambitious military operation; American public support for the war declined as a result of the Tet casualties and the ramping up of draft calls. Subsequently, the Johnson Administration sought negotiations to end the war, which were derailed in a secret agreement between then-former Vice President Richard Nixon, who planned to run as the Republican candidate in the 1968 United States presidential election, and South Vietnamese President Nguyễn Văn Thiệu.