The F-105 was no longer in production by 1964. With severe combat attrition of the F-105 inventory, the need for a more sophisticated aircraft resulted in the conversion of 36 F-4C Phantom II aircraft, designated F-4C Wild Weasel IV. The F-4C Wild Weasel IV also bore the unofficial designation of EF-4C.
In the spring of 1972 the U.S. renewed large-scale bombing of North Vietnam in response to the communist Easter Offensive against South Vietnam. Since the end of Rolling Thunder in 1968, the North Vietnamese had developed their fighter forces and created an integrated defense system of mutually supporting early warning radar, AAA and SAM sites. By 1972 the North Vietnamese had more than 200 SA-2 launchers, extending their coverage into parts of South Vietnam.
To assist the 17th Wild Weasel Squadron, the USAF sent over part of the F-105G-equipped 561st Tactical Fighter Squadron — together they totaled 28 F-105 Wild Weasel aircraft. In addition to these, the USAF introduced the F-4C Wild Weasel IV to combat during Linebacker.
Efforts to make the F-4C a Wild Weasel aircraft started in 1966, but developmental problems prevented it from being fielded until the spring of 1969. One F-4C Wild Weasel unit, the 67th TFS at Kadena Air Base, Okinawa, sent six F-4C Wild Weasels to Korat, Thailand, in September 1972. Used in southern North Vietnam during Linebacker, the F-4C Wild Weasel aircrews effectively supported strikes around Hanoi during Linebacker II, flying 460 sorties in Southeast Asia without loss.
Linebacker II, the most concentrated air campaign against North Vietnam, began on Dec. 18, 1972, after the communists again stalled at the peace table. Over half the targets were within 25 miles of Hanoi, protected by some of the densest air defenses in the world. Even so, the Wild Weasels, along with other anti-SAM measures, kept SA-2 losses relatively low, without any losses to themselves. In 1972, the year of Linebacker and Linebacker II, the North Vietnamese fired over 4,000 SA-2s — nearly half the total they fired during the entire war — shooting down 49 U.S. aircraft (meaning it took over 80 SA-2s to down one aircraft).
