On the one hand, we had various kinds of warning. The bombing of the federal building in Oklahoma City in 1995 and the Tokyo nerve-gas attack that same year provided a powerful demonstration of some of the methods terrorists might employ and the destruction they could achieve. It was also abundantly clear, from a decade-long series of lethal attacks on U.S. facilities abroad, and an attempt to knock down the World Trade Center in 1993, that a group or groups of terrorists were making a determined effort to strike at the U.S. That these same terrorists might attempt once again to hit targets in the American homeland was an obvious possibility; indeed, the CIA issued a series of generalized alerts to that effect, the most recent appearing this past August.