The USAF had deemphasized ACM since most air combat doctrine since the late 1950s, centered around delivering nuclear weapons over Europe, or firing missiles at beyond visual range at bombers, not daylight dogfighting, which was thought to be obsolete in the missile age. The primary US fighter used against North Vietnamese MiGs, the F-4 Phantom, did not even have an internal gun. Yet, US pilots were finding themselves hard-pressed to prevail over the nimble VPAF MiGs and by late 1966, they had grown to be a real threat to US aircraft operating over the North.
Even more vexing were Rules of engagement (ROE) that did not even permit beyond-visual-range (BVR) firing of missiles. Radar-guided Sparrows rarely worked, and the short range Sidewinder was ineffective in many dogfighting maneuvering situations. It was found that Phantom training against other Phantoms did not reflect the reality of a target that was smaller, smokeless, and more agile. Ever since the success of the American Volunteer Group (Flying Tigers), aerial tacticians have advocated exploiting known differences in aircraft to maximize one's own advantages while minimizing the disadvantages of one's own platform, thus neutralizing the superior maneuverability and climbing speed of, for example, a Zero compared to the rugged, fast-diving and powerfully armed P-40 Tomahawk. However, US pilots found themselves the victims of VPAF MiG-21s using Chennault's Flying Tiger tactics of hit and run attacks successfully against them.
In 1968, the Navy took a hard look at its air-to-air problems over North Vietnam and tasked Captain Frank Ault to come up with recommendations to improve the situation. His report became known as the Ault Report and it resulted in establishment of TOPGUN and incorporation of DACT into the syllabus. The United States Navy Fighter Weapons School adopted the nimble subsonic A-4 Skyhawk to simulate subsonic Soviet fighters, while the F-5E Tiger simulated the supersonic MiG-21 Fishbed fighter. Both the Skyhawk and Tiger were used in the 1986 film Top Gun. After aerial combat resumed again in 1972 over North Vietnam, the Navy had scores of TOPGUN graduates and were ready to take on the VPAF MiG-17, MiG-19 and MiG-21 pilots that had also been training and were prepared for the resumption of hostilities. The Navy's exchange ratio soared to over 20:1 before loss of a Marine Phantom brought it to 12.5:1 by 1973; an unqualified testament to the value of the TOPGUN approach and DACT. The USAF did not improve its exchange ratio at all in the same timeframe and hurriedly began to adopt DACT even inviting Navy F-8 Crusaders to visit a base in Thailand in 1972 to conduct DACT with the F-4 Phantoms based there.
The A-4 Skyhawk has since been replaced by the T-45 Goshawk, a navalized British Hawk trainer. F-16s have been used to simulate later generation Soviet fighters such as the MiG-29. The now-retired F-14 Tomcat was also used in various paint schemes to simulate Iranian F-14s, as well as the large Su-27 Flanker. The Air Force has reportedly also used actual captured or purchased Soviet fighters on occasion. Some have complained that not enough new instructor pilots are being trained, and that the maintenance situation on many aircraft, especially the Tomcat, was very poor.