In air-to-air combat, the unseen opponent is the greatest threat. An enemy pilot who slips onto your tail, your "six o'clock," is the one who is most likely to shoot you down. The classic fighter dictum, "keep checking six," may be as applicable on the ramp and on the runways as it is at 20,000 feet. This report explores the possibility that future adversaries will use ground attacks on U.S. Air Force (USAF) bases as at least a partial countermeasure to overwhelming U.S. air superiority. It also identifies, in broad terms, the types of initiatives that have the most potential to counter this evolving threat. The means, motives, and opportunity for ground attacks are converging to create a worsening ground threat to USAF air bases. Base vulnerability will be exacerbated by the kinds of expeditionary operations that are likely to be the most common military action in the future. Standoff attacks from perhaps several miles outside the base's perimeter pose the greatest danger, a danger that is amplified by the ongoing diffusion of affordable-yet-sophisticated weapon technologies and military gear. Surveillance and detection of enemy attack teams well outside the base perimeter will be key to protecting the base, its assets, and its personnel. The authors suggest that increasing the capabilities of the Security Police (SP) against the standoff threat will require some changes in USAF training policies and utilization of passive measures (e.g., deception, camouflage, and hardening) to protect key USAF assets. Finally, whereas defense of air bases against ground attack has been traditionally viewed within the USAF as an SP problem, the authors judge that it should be more properly seen as a challenge to airpower itself, since without secure bases, USAF operations could be severely impeded.