U.S. Air Force efforts to retire older fighters (137 F-15s and 177 F-16s) early, to provide more money to buy new F-22s and F-35s will cause several National Guard interceptor units to be disbanded. This, in turn, will eliminate fighters, available for 24 hour air defense alert, in many parts of the country. Currently, 16 of 18 air bases that provide these air defense fighters use National Guard fighters and pilots.
NORAD (North American Aerospace Defense Command) has been in charge of defending U.S. air space for over half a century. During the Cold War, the mission was to take down bombers from the Soviet Union, or Cuba. A secondary mission was to go up and check out commercial aircraft that were in some kind of trouble, or flying in restricted (for military use, usually) air space. After September 11, 2001, that mission became more important. In the last two decades, fighters have also been sent up after suspected drug smugglers. From the 1950s into the early 1960s, the U.S. Army also had anti-aircraft missile units stationed around major metropolitan areas.
The air force believes that it can handle the mission even with the early retirements. But a recent government report suggested otherwise, and now it's a political issue. Politicians are loathe to take the heat from voters alarmed at losing their air cover. In any event, this won't happen for another decade, and the air force is hoping that the political and media outcry will die down before then.