Although only eight US aircraft had been destroyed by SAMs up to the end of November 1965, the growing threat posed by the SA-2 was taken very seriously.  Soon after the first SAM was fired in North Vietnam, the USAF formed a task force to devise a way to counter the new threat.  It was quickly established that the weakest point of the SA-2 system was the missile’s reliance on its Fan Song target-tracking radar that was usually situated within or very close to the star-shaped missile battery site.  One way to counter the SAM threat was to jam the target acquisition and tracking radar using modified RB-66 aircraft.  However, it was also realised that a more aggressive approach was needed and the USAF task force put forward another solution.

The result of the investigation was a program that became known as Wild Weasel I and involved the use of two-seat F-100Fs modified to carry newly-developed avionics.  The system consisted primarily of the AN/APR-25 radar homing and warning receiver which gave a warning and the bearing of an active enemy radar; the AN/APR-26 tuned crystal receiver which detected a missile launch as the strength of the SA-2’s guidance signal increased just before lift off; and the IR-133 panoramic receiver which gave a longer range indication of the type of threat by signal analysis.  The receivers could also detect whether the radar was either SAM, AAA or early warning but none of the equipment could give an indication of range from the enemy radar.

The first four of a total of seven F-100Fs were modified and delivered to Eglin AFB where the system was to be tested in early September 1965.  The original five Weasel crews consisted of F-100 pilots together with electronic warfare officers (who were usually known simply as ‘EWOs’ or ‘Bears’) from the B-52 and B-66 communities.  As the Weasel mission depended very much on teamwork the crews were allowed to team up and usually flew together.  After several weeks of testing and training the four aircraft left Eglin for Korat on 21 November, arriving there four days later.  The unit was still regarded as being a test unit and was attached to the 6234th TFW at Korat.

The Wild Weasels flew their first operational mission on 1 December when two F-100Fs led a flight of F-105s to a target in North Vietnam.  The Weasels flew either as part of a Rolling Thunder strike force or, usually accompanied by a small number of strike aircraft, on search and destroy missions not necessarily connected to any ongoing strike.  The Weasels usually flew some minutes ahead of the strike force with the EWO listening for any indication that their aircraft had been illuminated by a SAM radar.  When a radar was heard the EWO vectored the pilot towards it.  If the Weasel could get close enough to the missile site to see it before a missile was launched, it would attack the site (aiming, if possible, for the radar antenna or control van) thereby marking it for the accompanying flight of F-105s to complete the destruction with bombs or napalm.  If, however, a missile was launched before the site could be attacked, the Weasel pilot used a manoeuvre that was developed early on and usually worked.  If the pilot could see the missile tracking towards him, he waited until he judged that it was close enough and then broke hard towards it in a diving turn.  This manoeuvre was usually sufficient to break the missile radar’s lock on the target which could not then be re-acquired in time before the SA-2 passed the aircraft and exploded way beyond it or on the ground.  In the early Weasel missions the F-100Fs carried LAU-3 rockets and napalm but later it was found that 500lb or 750lb bombs or CBUs were the most effective weapons.

The Wild Weasels got their first confirmed kill against a SAM site hidden in a village near the Red River on 22 December during a Rolling Thunder strike on a railway yard at Yen Bai.  Three more F-100Fs arrived at Korat in February 1966 and two months later the first AGM-45A Shrike anti-radiation missiles were received.  The Navy-developed Shrike used a passive homing receiver to ride the SAM’s radar beam back to its source, the Fan Song radar, which it then destroyed.  The first Shrike was fired by the USAF on 18 April 1966 but its effective range of 10-12 miles limited its effectiveness and it was eventually replaced by the Standard ARM.  The US Navy had been fitting some of its Iron Hand Skyhawks with Shrikes for about year by this time.  VA-23 was the first Navy squadron to fire the Shrike on 23 April 1965 but the A-4 lacked the specialist avionics that the Weasels carried.