From George C. Herring, America’s Longest War: The United States and Vietnam, 1950-1975, pp. 179, 180:
The United States relied heavily on bombing. Airpower doctrine emphasized that the destruction of an enemy’s war-making capacity would force that enemy to come to terms. The limited success of strategic bombing as applied on a large scale in World War II and on a more restricted scale in Korea raised serious questions about the validity of this assumption. The conditions prevailing in Vietnam, a primitive country with few crucial targets, might have suggested even more questions. The air force and navy advanced unrealistic expectations about what airpower might accomplish, however, and clung to them long after experience had proven them unjustified. The civilian leadership accepted the military’s arguments, at least to a point, because bombing was cheaper in lives lost and therefore more palatable at home, and because it seemed to offer a quick and comparatively easy solution to a complex problem. Initiated in early 1965 as much from the lack of alternatives as from anything else, the bombing of North Vietnam was expanded over the next two years in the vain hope that it would check infiltration into the South and force North Vietnam to the conference table. …
The manner in which airpower was used in Vietnam virtually ensured that it would not achieve its objectives. Whether, as the Joint Chiefs argued, a massive, unrestricted air war would have worked remains much in doubt. In fact, the United States had destroyed most major targets by 1967 with no demonstrable effect on the war. Nevertheless, the administration’s gradualist approach gave Hanoi time to construct an air defense system, protect its vital resources, and develop alternative modes of transportation. Gradualism in encouraged the North Vietnamese to persist despite the damage inflicted on them.