At midday on 30 March 1972, 30,000 PAVN troops, supported by regiments of tanks and artillery, rolled southward across the Demilitarized Zone (DMZ) that separated the two Vietnams.

[13] Although the conventional attack by the PAVN, which included the extensive use of armor and heavy artillery, riveted the attention of the allies on the northern provinces, it was only the first of three such operations that were launched that spring.

[15] On 12 April, the PAVN struck again, this time moving in from eastern Laos and seizing a series of border outposts around Đắk Tô in Kon Tum Province in the Central Highlands.

Hanoi had initiated the offensive to coincide with the winter monsoon, when continuous rain and low cloud cover made air support difficult.

President Richard Nixon's first response was to consider a three-day attack by Boeing B-52 Stratofortress heavy bombers on Hanoi and the port city of Haiphong.

His National Security Advisor, Dr. Henry Kissinger, convinced Nixon to reconsider, since he did not want to jeopardize the formalization of the Strategic Arms Limitation Treaty (SALT I) with the Soviet Union, that was due to be signed in May.

[22] To prevent a total ARVN collapse and to protect American prestige during the upcoming summit meeting with Soviet general secretary Leonid Brezhnev, Nixon decided to risk a massive escalation of force.

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.

[22] 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.

[22] Between 6 and 15 April, U.S. aircraft also struck and destroyed the Paul Doumer and Thanh Hóa bridges and the Yên Viên railway marshalling yard.

[34] 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".

Unwilling to jeopardize the normalisation of relations with the West and wary of Washington's growing relationship with Beijing, Brezhnev agreed to apply pressure to Hanoi to end the offensive and negotiate seriously.

Due to conflicting orders from their high command, ARVN units joined an exodus of refugees heading southwards, abandoning Quảng Trị city.

On 5 May, Nixon ordered the Joint Chiefs to prepare to execute the aerial mining portion of the Duck Hook plan within three days under the operational title Pocket Money.

[42] At precisely 09:00 (local time) on 9 May, six US Navy A-7 Corsair IIs and three A-6 Intruders from the USS Coral Sea flew into Haiphong harbor and dropped thirty-six 1,000-pound (450 kg) Mark-52 and Mark-55 naval mines into its waters.

The reason for the precise timing of the strike became apparent when Nixon simultaneously delivered a televised speech explaining the escalation to the American people: "The only way to stop the killing is to take the weapons of war out of the hands of the international outlaws of North Vietnam.".

Although Moscow and Beijing publicly denounced the American operation, they were not willing to jeopardize their thawing relationship with the U.S. and Hanoi's requests for support and aid from its socialist allies met with only cool responses.

[44] Operation Linebacker, the code name for the new interdiction campaign, would have four objectives: to isolate North Vietnam from its sources of supply by destroying railroad bridges and rolling stock in and around Hanoi and north-eastwards toward the Chinese frontier; the targeting of primary storage areas and marshalling yards; to destroy storage and transshipment points and to eliminate (or at least damage) the North's air defense system.

[47] With nearly 85 percent of North Vietnam's imports (which arrived by sea) blocked by Pocket Money, the administration and the Pentagon believed that this would cut its final lines of communication with its socialist allies.

[53] The North was feeling the pressure, admitting in the official PAVN history that "between May and June only 30 percent of supplies called for in our plan actually reached the front-line units.

Navy pilots, employing a mutually supporting "loose deuce" tactical formation and many with TOPGUN training, claimed a kill ratio of 6:1 in their favor in May and June, such that after that the VPAF rarely engaged them thereafter.

[58] During August the introduction of real-time early warning systems, increased aircrew combat experience and degraded VPAF ground control interception capabilities reversed the trend to a more favorable, with 4:1 kill ratio were claimed.

[62] Although Linebacker was largely carried out by air, naval forces were also deployed to provide counter-battery fire against enemy targets along the coast and other important logistical areas and in support of ground troops.

[63][64] The stalled offensive in the South and the devastation in North Vietnam had helped to convince Hanoi to return to the bargaining table by early August.

Gone were Hanoi's demands for the ouster of South Vietnamese President Nguyễn Văn Thiệu and his replacement by a coalition government in which the Viet Cong would participate.

"[67] It succeeded, where Rolling Thunder had failed, he claimed, for three reasons: Nixon was decisive in his actions and gave the military greater latitude in targeting; American airpower was forcefully and appropriately used; and the immense difference in the technology utilized made Linebacker the first bombing campaign in a "new era" of aerial warfare.

[68] During and immediately following the PAVN offensive, U.S. Air Force, Navy and Marine Corps aviators had flown 18,000 sorties in the four northern provinces of South Vietnam and dropped 40,000 tons of ordnance in the Battle of An Lộc.

[70] From the beginning of Freedom Train in April to the end of June 1972 the United States lost 52 aircraft over North Vietnam: 17 to missiles; 11 to anti-aircraft weapons; three to small arms fire; 14 to MiGs; and seven to unknown causes.

[75] USAF General Robert N. Ginsburgh, of the Office of the Secretary of the Air Force, summed up the attitudes of U.S. commanders by remarking that Linebacker had "a greater impact in its first four months of operation than Rolling Thunder had in three and one-half years.