Clarke says the US set up point defenses against air attack for 5-6 events starting with the Atlanta Olympics. These involved things like a guy or two with a Stinger missle on a tall building near the specific site being protected and an increased level of air defense alert. It was expensive (notice they quit flying jets over NY after a few weeks?) and hardly fool proof and was never carried out too long or to protect more than a few buildings within a city. Clarke had suggested setting up protection for the White House and Congress. He admits that wouldn't have helped the Twin Towers or the Pentagon. Too many buildings in the US for that kind of point defense to protect everything.