F-106 fighter-interceptors; ground-based warning radars; and associated bases and personnel) subsequently transferred to TAC under a sub-entity named Air Defense, Tactical Air Command (ADTAC). ADC's EC-121 Warning Star airborne early warning aircraft had already been retired by 1978 and it never received that aircraft's replacement, the E-3 Sentry AWACS, that began entering the TAC inventory in 1977. As a result of changes in USAF doctrine and tactics, AWACS was always intended as strictly a TAC airframe nominally operating from forward-deployed locations and incorporating many of the lessons learned from employing the EC-121 in its Big Eye, College Eye, Rivet Top and Disco roles with PACAF's 7th Air Force during the Vietnam War. The new command was, essentially, a transition organization between ADC, and the transfer of the continental air defense mission from a combination of the Regular Air Force and the Air National Guard to one totally residing in TAC-gained assets of the Air National Guard by 1990. Red Flag Aggressor training The 57th Fighter Weapons Wing at Nellis AFB, Nevada began giving Regular Air Force units some of the most intense combat training ever achieved through the Red Flag program, beginning in 1976. This would later expand to include the Air Force Reserve, the Air National Guard, aviation squadrons of the U.S. Navy and U.S. Marine Corps (augmenting their Naval Fighter Weapons School / TOPGUN program), and various NATO and Allied nations. The origin of Red Flag was the unacceptable performance of U.S. Air Force pilots and weapon systems officers in air combat maneuvering (ACM) (air-to-air combat) during the Vietnam War in comparison to previous wars. Air combat over North Vietnam between 1965 and 1973 led to an overall exchange ratio (ratio of enemy aircraft shot down to the number of own aircraft lost to enemy fighters) of 2.2:1. In fact, for a period of time in June and July 1972 during Operation Linebacker, the ratio was less than 1:1. The aggressor units at Red Flag were originally equipped with readily available T-38 Talon aircraft loaned from the Air Training Command (ATC) to simulate the Soviet Union's MiG-21. Northrop F-5 Tiger II fighters, painted in color schemes commonly found on Soviet and Warsaw Pact aircraft, were added shortly thereafter and became the mainstay until the F-16 was introduced in the mid/late 1980s. The Red Flag exercises, conducted in four to six cycles per year by the 414th Combat Training