There's a lot to it: yes, Vietnam War air-to-air, also economics (cheaper to buy 1000s of F-5s than 100s of F-15s), I think there was a cultural preference for lightweight, visual-only day fighters, but they also thought it'd be more effective in combat b/c of tests in late 70s

In 1976-77, the AIMVAL/ACEVAL tests ran hundreds of scenarios of F-15s vs F-5s, partly to test the planes, partly to test new missiles (especially the AIM-9L all-aspect Sidewinder). This test was to the reform mvmt what the Ostfriesland tests were to Mitchell

What happened was that the F-15s dominated everything--unless the total number of planes in the air got higher. Once you got towards 6 or 8 planes in one big dogfight, the exchange ratio started to even out