The U.S. Senate Foreign Relations Committee begins hearings to investigate American policy in Vietnam. This was a direct result of the Tet Offensive, in which Viet Cong forces, supported by large numbers of North Vietnamese troops, launched the largest and best-coordinated offensive of the war. During the attack, the Viet Cong drove into the center of South Vietnam's seven largest cities and attacked 30 provincial capitals ranging from the Delta to the DMZ.
Efforts to assess the offensive's impact began well before the fighting officially ended. Militarily, Tet was decidedly an Allied victory, but psychologically and politically, it was a disaster. The offensive had indeed been a crushing military defeat for the Viet Cong and the North Vietnamese, but the size and scope of the communist attacks had caught the American and South Vietnamese allies completely 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. This disenchantment caused congressional opponents to call for hearings.
Early sessions in the congressional hearings focused on the 1964 Gulf of Tonkin incident, which had led to the passage of the Tonkin Gulf Resolution, the legal basis for Johnson's escalation of the war. Senators William Fulbright (D-Arkansas) and Wayne Morse (D-Oregon) charged that the Defense Department had withheld information on U.S. naval activities in the Gulf that provoked North Vietnam, leading to the charge of a "credibility gap." At issue was whether the administration had provided Congress with truthful data at the time it was seeking passage of the Tonkin Gulf Resolution in August 1964, which had considerably broadened the president's war-making authority in Southeast Asia. There was no firm resolution of the charges, but the debate reached a new intensity when the New York Times reported that General William Westmoreland, U.S. commander in Saigon, had requested another 206,000 troops. The possibility of another major troop increase provoked a stormy reaction in Congress--both Democrats and Republicans demanded an explanation and insisted that Congress share in any decision to expand the war. In March, 139 members of the House of Representatives sponsored a resolution calling for a full review of American policy in Vietnam.
Eventually the Tet Offensive and the subsequent congressional reaction helped convince Johnson, who was frustrated with his inability to reach a solution in Vietnam, to announce that he would neither seek nor accept the nomination of his party for president.