Tet offensive, 1968, a series of crucial battles in the Vietnam War. On Jan. 31, 1968, the first day of the celebration of the lunar new year, Vietnam's most important holiday, the Vietnamese Communists launched a major offensive throughout South Vietnam. It took weeks for U.S. and South Vietnamese troops to retake all of the captured cities, including the former imperial capital of Hue. Although the offensive was not militarily successful for the Vietnamese Communists, it was a political and psychological victory for them. It dramatically contradicted optimistic claims by the U.S. government that the war had already been won.