However, real-world experience in the Vietnam War revealed some shortcomings in American fighter capabilities, as early generation Soviet jet fighters proved to be more of a challenge than expected for U.S. designs. Although U.S. pilots had achieved favorable kill-to-loss ratios, combat had revealed that air-to-air missiles (AAM) of this era were significantly less reliable than anticipated. Furthermore, the rules of engagement in Vietnam precluded long-range missile attacks in most instances, as visual identification was normally required. Under these conditions, combat invariably closed to short ranges where maneuverability and short-range air-to-air weapons became critical, even for dedicated interceptors like the Convair F-102 Delta Dagger.