Operation LINEBACKER II
Executed between December 18 and 29, 1972, Operation LINEBACKER II was the most intensive bombing campaign taken by the United States against North Vietnam, taken after the withdrawal of U.S. ground troops from the South, and intended to force the North back to the bargaining table at the Paris Peace Talks. Ordered by Richard Nixon, it lasted for 11 days, and had the desired result of restarting negotiations.
The operation was significant at multiple levels of military doctrine. It was radically different both in the message being sent to the North Vietnamese leadership, and in both very different levels of force and the means of applying it, than Operation ROLLING THUNDER under the Johnson Administration. Where Robert McNamara had been Johnson's key adviser on the use of force, Henry Kissinger, with a very different set of geostrategic assumptions, had an equally key role with Nixon.
Operation LINEBACKER I, carried out from March to October, also was a radical change from the ROLLING THUNDER approach, but it had different objectives. Its goals were at a more military strategic level, of interfering with the North Vietnamese invasion of the South.
At the level of grand strategy, it specifically used military means in support of diplomacy. It was focused on compellence to achieve a specific, short-term objective, rather than either fundamentally disrupting the enemy by attacking a center of gravity, or using force and the threat of force to "signal" intentions.
While Clark described Linebacker II as a "strategic" success, it actually achieved grand strategic success as well. Nixon had given North Vietnam a 72-hour ultimatum, but they appear not to have appreciated the potential effect of a nearly unlimited, even though somewhat flawed, air campaign.
Sir Robert Thompson, British commander of the successful counterinsurgency in Malaya, said "In my view, on 30 December 1972, after eleven days of those B-52 attacks on the Hanoi area, you had won the war. It was over… They and their whole rear base at that point were at your mercy. They would have taken any terms."
Linebacker II had military as well as broader objectives. Where Linebacker I had the operational objective of stopping the conventional invasion of the south, Linebacker II was focused on destroying the northern logistical infrastructure that backed infiltration and invasion. It was fully understood, however, that destroying the internal and external transportation systems of the DRV were in a very different context than attacking the equivalent Second World War systems of Germany and Japan.
While the enemy could, given time, repair the damage, in WWII, the enemy would be permanently stopped by being conquered on the ground. Invasion and occupation of North Vietnam was not being considered, so they would, eventually, rebuild. Also, the truly important industrial base, destroyed in Germany and Japan, was in the Soviet Union and China, and could not be attacked. It was realistic, however, to break the transportation system between the suppliers and the operational support infrastructure in North Vietnam.
By focusing on infrastructure in North Vietnam, while the defenses were stronger than those of the forces in the South, targets were fixed and generally well-identified. As in Linebacker I, the rules of engagement permitted serious suppression of enemy air defense.
A serious operational problem, hardly new to the war, was divided command and lack of coordination. Lessons learned would point to inadequate operational level planning concerning orders issued to aviators which specified flying stereotyped ingress and egress routes. Unity of command was effectively nonexistent, with authority divided among United States Pacific Command, Strategic Air Command (SAC), carrier aviation (Task Force 77, United States Seventh Fleet), and Military Assistance Command, Vietnam 
In particular, SAC culture did not envision using B-52s, against a serious defense, in a joint command. Perhaps SAC commanders had become complacent about B-52s in Southeast Asia, due to the lack of defense against the ARC LIGHT strikes in the South. Still, one can look at SAC resistance to the development of the Single Integrated Operational Plan for nuclear warfare, and, even there, less than ideal cooperation with the Navy. Just as LINEBACKER II was a radical change over ROLLING THUNDER, it is illustrative to compare LINEBACKER II to Operation DESERT STORM, and the continuing evolution of the joint air tasking order.
Clark usefully separated the tactical phases by the level of B-52 losses, and the changes of tactics to reduce those losses.  Initial losses were unacceptable, but dropped significantly with tactical change in the second phase, and, by the third phase, North Vietnamese essentially had no remaining air defense.
In the campaign, over 20,000 tons of bombs were dropped. All rail transportation, in a 10 mile radius of Hanoi, was disrupted. North Vietnam lost 1/4 of its fuel, 80% of its electrical generating capacity, and saw its external supply volume drop from 160,000 tons per month to 30,000 tons. It signed a peace agreement one month after the start of the operation. 
In the initial phase, the initial strikes were at night, by three waves of a total of 129 B-52s, flying from Thailand and Guam. On the first night, they attacked five targets. Night attack was chosen to minimize the threat from fighters, although surface-to-air missiles (SAM) were, in retrospect, the real threat. The North Vietnamese had a substantial supply of S-75 Dvina Soviet-built missiles (Western designation: SA-2 GUIDELINE) As opposed to the intense SEAD campaign in Desert Storm, there was only limited, by modern standards, attacks by F-111 all-weather attack aircraft, against the North Vietnamese integrated air defense system.
While the B-52s did have EB-66, EA-3, and EA-6B Prowler escorting electronic warfare aircraft, as well as F-105 aircraft armed with early anti-radiation missiles, there was a great dependence on chaff dropped by F-4 Phantom II fighters, as well as the onboard B-52 jamming systems.
As had long been the case, a Soviet intelligence ship was stationed in international waters off Guam, and reported the B-52 takeoffs. Still, it was 5,500 miles from Guam to the target, so the takeoff warnings, at least on the first night, did not tell the North Vietnamese that the target was Hanoi. The worst losses were on the second night, against a defense that understood the tactics that were repeated throughout phase I.
Once in the target areas, NVAF fighters did not attack the B-52s, but assisted the SAM crews by following the bomber formations and reporting course, altitude, and speed. Chaff was not enough to hide the immense radar target of B-52 formations. At best, it degraded North Vietnamese radar. At worst, the chaff marked the approach corridors, 5 miles wide and 100 miles long, which were repeated on the next two nights. The waves of bombers stretched back for 70 miles, with hours between the first and last aircraft.
Using an unexpected but technically reasonable technique with their S-75 Dvina (Western designation SA-2 GUIDELINE SAMs, the North Vietnamese did not try to use their beam-riding radar to take the missiles all the way to the target. Instead, they fired barrages of missiles into the path of the bombers, relying on the proximity fuzes of the missiles for terminal guidance.  Nevertheless, the technique was not sustainable; an estimated 60 SAMs were fired for each B-52 kill.  U.S. jamming was designed to interfere with the SA-2 command guidance.
The most fundamental change in the second phase, between the 21st and 24th, was to send the B-52s in a single wave, with more support, and against more targets. Within the wave, the bombers flew varying courses. At the Christmas pause, Nixon told the North Vietnamese that there would be no further pauses until they capitulated.
Even though the third phase was even more tactically sophisticated, by the 26th, North Vietnam had exhausted its SAM supply, the facility for assembling more was destroyed, the fighter airbases were unusable, and IADS sensors and control were largely eliminated. North Vietnam indicated a willingness to negotiate.
After a Christmas pause, the third wave both compressed the time over target, reducing exposure and maximizing shock, as well as even more variation in approaches. At one point, 120 bombers, flying different courses, attacked in a 15 minute period. Chaff was used in clouds rather than corridors. Even so, there was very little defense.
By the 29th, not only had defenses been suppressed, virtually all significant targets were gone. 
- Clodfelter, Mark (1989), The Limits of Air Power: The American Bombing of North Vietnam, Simon and Schuster, ISBN 0029059909, p. 184
- Clark, Gregory S. (04 Feb 2002), Linebacker 2: Achieving Strategic Surprise, Naval War College
- Sir Robert Thompson, cited in Clark, pp. 8-9
- Clark, p. 10
- Littrell, Dennis R. (1998), LINEBACKER II: The December 1972 Air Campaign, Army War College, p. 24
- Clark, pp. 9-10
- Littrell, pp. 15-16
- Clark, p. 10
- Littrell, pp. 13-14
- Kopp, Carlo (November, 1986, March/May, 1987), "The Long Range Penetrator", Australian Aviation
- Littrell, pp. 14-15