For Americans the year 2001 will be remembered as being dominated by two acts of terrorism. The September 11, 2001, attacks involved hijacked jetliners used as bombs to destroy the World Trade Center in New York City and damage the Pentagon in Washington, DC. The other incident— the October and November mailings of letters containing deadly anthrax powder to media and political figures—involved the use of a biological agent. The two incidents marked a new era of U.S. vulnerability to terrorism within its borders.

The question of which incident is a more alarming harbinger of the future is a matter of some debate. The anthrax mailings ultimately killed only five people (and left eighteen others sick)—a much lower number of fatalities than the estimated three thousand people who perished on September 11. Some observers cite this figure...