The presidency of Lyndon Baines Johnson began on November 22, 1963, when Johnson became the 36th President of the United States upon the assassination of President John F. Kennedy, and ended on January 20, 1969. Johnson did not run for a second full term in the 1968 presidential election, he was succeeded by Republican Richard Nixon. His presidency marked the high tide of modern liberalism in the United States. The Tet Offensive convinced senior leaders of the Johnson administration, including the 'Wise Men' and new Defense Secretary Clark Clifford, that further escalation of troop levels would not help bring an end to the war. On March 31, 1968, Johnson announced that he would halt the bombing in North Vietnam, while at the same time announcing that he would not seek re-election. He also escalated U.S. military operations in South Vietnam in order to consolidate control of as much of the countryside as possible before the onset of serious peace talks.