In January 30, 1968, the Tet Offensive began in South Vietnam.
  

   
At dawn on the first day of the Tet holiday truce, Viet Cong forces--supported by large numbers of North Vietnamese troops--launch the largest and best coordinated offensive of the war, drivingg into the center of South Vietnam's seven largest cities and attacking 30 provincial capitals from the Delta to the DMZ.

Among the cities taken during the first four days of the offensive were Hue, Dalat, Kontum, and Quang Tri; in the north, all five provincial capitals were overrun. At the same time, enemy forces shelled numerous Allied airfields and bases. In Saigon, a 19-man Viet Cong suicide squad seized the U.S. Embassy and held it for six hours until an assault force of U.S. paratroopers landed by helicopter on the building's roof and routed them. Nearly 1,000 Viet Cong were believed to have infiltrated Saigon, and it took a week of intense fighting by an estimated 11,000 U.S. and South Vietnamese troops to dislodge them.   

 
Major South Vietnamese population centres and installations targeted by the Viet Cong during the 1968 Tet Offensive.
By February 10, the offensive was largely crushed, but with heavy casualties on both sides. The former Imperial capital of Hue took almost a month of savage house-to-house combat to regain. Efforts to assess the offensive's impact began well before the fighting ended. On February 2, President Johnson announced that the Viet Cong had suffered complete military defeat. General Westmoreland echoed that appraisal four days later in a statement declaring that Allied forces had killed more enemy troops in the previous seven days than the United States had lost in the entire war.    

The scene is Hue, during its darkest moment-the infamous TET offensive. U.S. marines are keeping low because of intense sniper fire from communists units which seized two-thirds of the ancient imperial Capital 2/4/1968. The Marines were pinned down behind this wall near the old citadel and radioed for support. U.S. spokesmen reported that leathernecks hauled down the North Vietnamese flag after seven days of fighting and recaptured the city. (UPI photo)  

Read more: http://www.upi.com/Archives/Audio/Events-of-1968/Vietnam-War/Tet-Offensive/#ixzz2rpo0yiO3 
Militarily, Tet was decidedly an Allied victory, but psychologically and politically, it was a disaster. The offensive was a crushing military defeat for the Viet Cong and the North Vietnamese, but the size and scope of the communist attacks caught the American and South Vietnamese allies by surprise. The early reporting of a smashing communist victory went largely uncorrected in the media and led to a psychological victory for the communists. The heavy U.S. and South Vietnamese casualties incurred during the offensive, coupled with the disillusionment over the earlier overly optimistic reports of progress in the war, accelerated the growing disenchantment with President Johnson's conduct of the war. Johnson, frustrated with his inability to reach a solution in Vietnam, announced on March 31, 1968, that he would neither seek nor accept the nomination of his party for  re-election.