The Tet Offensive was a military campaign during the Vietnam War. It was started on 30 January 1968 by forces of the Viet Cong and North Vietnam against South Vietnam, the United States and their allies.The purpose of it was to surprise the enemy with attacks against military and civilian commands and control centers in South Vietnam.t was started on 30 January 1968 by forces of the Viet Cong and North Vietnam against South Vietnam, the United States and their allies. The purpose of it was to surprise the enemy with attacks against military and civilian commands and control centers in South Vietnam.