Encouraged by the gradual withdrawal of US forces from Southeast Asia and the poor performance of South Vietnamese forces, the North Vietnamese general Vo Nguyen Giap planned an ambitious invasion of the South in an effort to bring the war to a victorious close.  Known as the Nguyen Hue Offensive to the North Vietnamese, and the Spring or Easter Invasion to the Americans, the offensive was launched on 30 March 1972 and took the South by surprise.  The North Vietnamese made three main thrusts into South Vietnam.  The largest and most successful of the North Vietnamese offensives took place in Military Region I where enemy troops swarmed across the DMZ and from Laos capturing Quang Tri and Hué and threatening Da Nang.  The second major invasion took place in the Central Highlands to the north and west of Pleiku where major battles were fought at Kontum and Dak To.  Lastly, Communist forces attempted to threaten Saigon itself by making a major thrust through Binh Long province to the north of the city.  However, after capturing Tay Ninh and the US airfield at Quan Loi the North Vietnamese were repulsed during the bitter siege of An Loc.  Unwilling to send ground troops back to Vietnam, President Nixon’s response was to use air power to blunt the offensive.  Reinforcements began to flood back into Southeast Asia within hours of the start of the offensive.  The first squadron to arrive was the 36th TFS which flew its F-4Ds from Osan AB, Korea to Da Nang to join the 366th TFW the day after the start of the Invasion.  The Marines despatched three F-4 squadrons to Da Nang in April and two A-4 squadrons to Bien Hoa in May.  The US Navy ordered four of its carriers to join the two already on duty at Yankee Station.  On 31 March the USAF had a total of 365 aircraft based in Southeast Asia.  By the end of May this figure had increased to 501 and, when all six carriers had arrived, the US Navy added another 400 combat aircraft available for operations.

The next series of USAF deployments from the USA commenced in early April under the code name Constant Guard.  For the record the Constant Guard I deployment commenced on 7 April and consisted of the 561st TFS (F-105G) to Korat, 334th TFS and 336th TFS (F-4E) to Ubon, and eight EB-66s from the 39th TEWS to Korat to supplement the squadrons there.  Constant Guard II started on 1 May and saw the deployment of the 58th TFS and 308th TFS (F-4E) to Udorn.  Constant Guard III was the largest of the deployments and involved the move of all four squadrons of the 49th TFW (F-4D) from Holloman AFB, New Mexico to Takhli, starting 7 May.  Constant Guard IV started on 13 May and involved the 36th TAS and 61st TAS (C-130E) deploying to the 374th TAW at Ching Chuang Kang AB, Taiwan.  Constant Guard V saw the return of the F-111A to Southeast Asia with the 429th TFS and 430th TFS moving to Takhli on 27 September.  Finally, Constant Guard VI involved the deployment of three squadrons of the 354th TFW with their new A-7Ds to Korat.  In conjunction with the move of tactical aircraft in Constant Guard, SAC also began a series of deployments of B-52s to Southeast Asia under the code name Bullet Shot that resulted in a total of 153 bombers deploying to Andersen and another eight to U-Tapao.