Editing Operation Linebacker (section)

==Build-up and air attacks==
American and [[Republic of Vietnam Air Force]] (RVNAF) aircraft had been supporting the defense (weather permitting) since the beginning of the offensive. These strikes were conducted in support of ARVN forces, and included those of the air wings of the carriers {{USS|Coral Sea|CV-43|6}} and {{USS|Hancock|CV-19|6}}. The bad weather limited the ability of the U.S. aircraft to assist in stemming the North Vietnamese onslaught. By 6 April, at naval and air bases around the globe, American forces were put on alert and ships and aircraft squadrons began moving toward Southeast Asia.<ref>{{Cite book |last=Morrocco |first=John |year=1985 |title=Rain of Fire: Air War, 1969–1973 |url-access=registration |url=https://archive.org/details/rainoffire00time |location=Boston |publisher=Boston Publishing Company |isbn=0-939526-14-X|page=170}}</ref>

The U.S. began a rapid build-up of airpower. The USAF deployed 176 F-4s and 12 [[Republic F-105 Thunderchief|F-105 Thunderchiefs]] from bases in the [[Republic of Korea]] and the U.S. to Thailand between 1 April and 11 May in Operation Constant Guard.<ref>Lavalle, pp. 19, 23–25. Also see Morocco, pp. 108–109.</ref> [[Strategic Air Command]] (SAC) dispatched 124 B-52s from the U.S. to Guam bringing the total B-52 strength available for operations to 209.<ref>Tilford, p. 224.</ref> The USN cut short its port period for the carriers {{USS|Kitty Hawk|CV-63|6}} and {{USS|Constellation|CV-64|6}} and ordered {{USS|Midway|CV-41|6}}, {{USS|America|CV-66|6}} and {{USS|Saratoga|CV-60|6}} to augment the fleet so that four or more carrier air wings could conduct missions simultaneously. [[7th Fleet]] assets in local waters were thereby increased from 84 to 138 ships.<ref name="Fulghum and Maitland, p. 142"/>

USAF tactical strikes against North Vietnam north of the [[20th parallel north|20th parallel]] were authorized on 5 April under the nickname ''Freedom Train''.<ref name="Tilford, p. 228"/> The first mass B-52 raid directed against the north was conducted on 10 April when 12 B-52s, supported by 53 attack aircraft, struck petroleum storage facilities around [[Vinh]].<ref>Wayne ''To Hanoi and Back''. Washington, D.C.: Smithsonian Institution Press, 2000, p. 225.</ref> By 12 April, Nixon had informed Kissinger that he had decided on a more comprehensive bombing campaign which would include strikes against both Hanoi and Haiphong.<ref name="Fulghum and Maitland, p. 142"/>

[[File:North Vietnamese Antiaircraft Weapons.jpg|thumb|right|North Vietnamese anti-aircraft defense weapons]]
The following day 18 B-52s struck [[Thanh Hóa]]'s [[Tho Xuan Airport|Bai Thuong Air Base]]. Three more days followed before another strike, this time by another 18 bombers in a pre-dawn attack against an oil tank farm outside Haiphong. They were followed by more than 100 tactical aircraft attacking targets around Hanoi and Haiphong during daylight.<ref name="Tilford, p. 228"/> Between 6 and 15 April, U.S. aircraft also struck and destroyed the [[Long Biên Bridge|Paul Doumer]] and [[Thanh Hóa Bridge|Thanh Hóa]] bridges and the [[Yên Viên Railway Station|Yên Viên railway marshalling yard]]. This marked the introduction of [[laser-guided bomb]]s against strategic targets in North Vietnam. Both bridges had previously been attacked unsuccessfully with conventional bombs and even missiles. The B-52s were then withdrawn from operations in the north and when they returned in June, their missions would be limited to South Vietnam.<ref>Thompson, p. 229.</ref>

By mid-month, nearly all of North Vietnam had been cleared for bombing raids for the first time in over three years. Air Force and Navy commanders and pilots were relieved that Nixon (unlike [[Lyndon B. Johnson|President Johnson]]) left the operational planning to local commanders and loosened the targeting restrictions that had hampered Operation Rolling Thunder.<ref>Stanley Karnow, ''Vietnam''. New York: Viking, 1983, p. 643.</ref> Between 1 May and 30 June B-52s, fighter-bombers, and gunships had flown 18,000 sorties against formidable anti-aircraft defenses with the loss of 29 aircraft.<ref>Casey, Dougan, Lipsman, p. 39.</ref>

The U.S. also now began what North Vietnamese historians have described as "using devious political and diplomatic schemes...to cut back the amount of aid being supplied to us by socialist nations".<ref>Van Thai & Van Quang 2002, p. 299.</ref> On 20 April Kissinger met secretly with Brezhnev in Moscow. Unwilling to jeopardize the normalisation of relations with the West and wary of Washington's [[China–United States relations#Rapprochement|growing relationship]] with [[Beijing]], Brezhnev agreed to apply pressure to Hanoi to end the offensive and negotiate seriously.<ref>On 21 February 1972 [[1972 visit by Richard Nixon to China|Nixon had landed in Beijing]] for his dramatic diplomatic breakthrough with the [[People's Republic of China]]. The Chinese, who had previously hoped that a long war in Southeast Asia would bleed both the Americans and their Vietnamese neighbors, now feared that a decline in American power would deprive them of a counterweight to the Soviet Union. Karnow, p. 638.</ref>

Brezhnev then arranged for another secret meeting between Kissinger and Hanoi's lead negotiator [[Le Duc Tho]], to be held on 2 May in Paris. On the day, the two men met for a session that Kissinger later described as "brutal and insulting."<ref>Fulghum and Maitland, p. 179.</ref> The North Vietnamese, sensing victory, were in no mood to make concessions. As a result of this meeting and the [[First Battle of Quảng Trị|fall of Quảng Trị City]] Nixon was prepared to up the ante, stating that "the bastards have never been bombed like they're going to be bombed this time".<ref>Fulghum and Maitland, p. 168.</ref>