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.