Operation Rolling Thunder was a frequently interrupted bombing campaign that began on 24 February 1965 and lasted until the end of October 1968. During this period U.S. Air Force and Navy aircraft engaged in a bombing campaign designed to force Ho Chi Minh to abandon his ambition to take over South Vietnam. The operation began primarily as a diplomatic signal to impress Hanoi with America's determination, essentially a warning that the violence would escalate until Ho Chi Minh "blinked," and secondly it was intended to bolster the sagging morale of the South Vietnamese.
The Johnson administration also imposed strict limits on the targets that could be attacked, for China and the Soviet Union were seen as defenders of communism who might intervene if the North Vietnamese faced defeat. Consequently, the administration tried to punish the North without provoking the two nations believed to be its protectors. In the view of the Air Force leadership, the campaign had no clear-cut objective nor did its authors have any real estimate of the cost of lives and aircraft.
General LeMay and others argued that military targets, rather than the enemy's resolve, should be attacked and that the blows should be rapid and sharp, with the impact felt immediately on the battlefield as well as by the political leadership in Hanoi. When Rolling Thunder failed to weaken the enemy's will after the first several weeks, the purpose of the campaign began to change. By the end of 1965, the Johnson administration still used air power as an attempt to change North Vietnamese policy, but bombing tended to be directed against the flow of men and supplies from the North, thus damaging the enemy militarily while warning him of the danger of greater destruction if he maintained the present aggressive course.
To persuade the North Vietnamese to negotiate, President Johnson restricted the bombing of North Vietnam to the southern part of the country on 31 March 1968, in effect, bringing Operation Rolling Thunder to an end. Preliminary discussions began in Paris in May but bogged down over trivial issues. In November, Johnson made another concession, ending the bombing throughout the north, and serious negotiations began in January 1969.
See the AFHSO publication by John Schlight: The War in South Vietnam: the Years of the Offensive, 1965-1968.
For additional information see the National Museum of the USAF.