US Air Force assumptions about future conflicts have proved to be in error. Since 1945 the Air Force has geared itself for air-to-air combat and a nuclear exchange with a major power. Although this is certainly America's most serious challenge, it has turned out to be the least likely one. For the reality of war has proved to be far different. Since World War II the US Air Force has fought in two wars against minor powers, used conventional weapons, and found its chief opposition to be ground-based air defense weapons. Not only did the nature of war prove different from the one anticipated, but the technology took a turn away from the offense to favor the defense. The big contributor to this shift in the balance between the offense and defense was the emergence of effective surface-to-air missiles. The airmen never appreciated the impact of ground-based air defense systems until it was too late. The US Air Force used ECM, direct action, and tactics to nullify the defensive threat but, in so doing and in relearning old lessons, suffered heavy losses. What should be emphasized to all American military personnel, especially all airmen, is that since World War I and especially since early 1944, US airmen have lost more aircraft in combat to ground-based air defense systems than to hostile aircraft. There is no indication that the future will be any different.