From Wikipedia

The 10,000 Day War
Episode Four

 Uneasy Allies

• Operation Rolling Thunder was the title of a gradual and sustained U.S. 2nd Air Division (later Seventh Air Force), U.S. Navy, and Republic of Vietnam Air Force (VNAF) aerial bombardment campaign conducted against the Democratic Republic of Vietnam (North Vietnam) from March 2nd, 1965 until November 1st, 1968, during the Vietnam War.

• The four objectives of the operation, (which evolved over time) were: To boost the sagging morale of the Saigon regime in the Republic of Vietnam; To persuade North Vietnam to cease its support for the communist insurgency in South Vietnam without actually taking any ground forces into communist North Vietnam; To destroy North Vietnam's transportation system, industrial base, and air defenses; and to interdict the flow of men and material into South Vietnam. Attainment of these objectives was made difficult by both the restraints imposed upon the U.S and its allies by Cold War exigencies and by the military aid and assistance received by North Vietnam from its communist allies, the Soviet Union and the People's Republic of China (PRC).

•  The operation became the most intense air/ground battle waged during the Cold War period, indeed, it was the most difficult such campaign fought by the U.S. Air Force since the aerial bombardment of Nazi Germany during World War II. Thanks to the efforts of its communist allies, North Vietnam fielded a potent mixture of sophisticated air-to-air and ground-to-air weapons that created one of the most effective air defense environments ever faced by American military aviators. After one of the longest aerial campaigns ever conducted by any nation, Rolling Thunder was terminated as a strategic failure in late 1968 having achieved none of its objectives.

• U.S. policy was for a time dictated by its perception of improvement in the Saigon government. No further commitment by the Americans would occur without tangible proof of the regime's survivability. Events in Vietnam, however, outraced this policy. By the beginning of 1965, it was stood upon its head - without further American action the Saigon government could not survive.

• Between March 1965 and November 1968, aircraft of the U.S. Air Force had flown 153,784 attack sorties against North Vietnam, while the Navy and Marine Corps had added another 152,399. On 31 December 1967, the Department of Defense announced that 864,000 tons of American bombs had been dropped on North Vietnam during Rolling Thunder, compared with 653,000 tons dropped during the entire Korean Conflict and 503,000 tons in the Pacific theater during the Second World War.