The shocking terrorist assault on September 11, 2001, was one of those “outliers” where, as Peter Bernstein might say, “the wildness lies.” In the immediate aftermath, the coordinated attacks on the Pentagon, on the World Trade Center, and in the air over Pennsylvania appeared to defy all logical explanation and rational analysis. The previous comparable attack by foreigners on continental United States soil occurred almost two centuries ago, when British soldiers burned District of Columbia government buildings during the War of 1812. Prior to the 1990s, the most serious act of domestic terrorism had arguably occurred more than a century earlier with John Brown's 1859 attack on a federal arsenal at Harper's Ferry.

Throughout the 1990s, America witnessed a growing frequency of large and small domestic and foreign terrorist incidents.