One of the major questions in the aftermath of the September 11, 2001, terrorist attacks on America is what effects the U.S. war on terror will have on Americans’ civil liberties. Within days of the attacks, some pundits and public officials began calling for expanded police, FBI, and CIA powers to combat future attacks. They were quickly answered by commentators who warned that to turn the United States into a police state would be to give the enemies of democracy a partial victory. “If we are intimidated to the point of restricting our freedoms, the terrorists will have won,” said American Civil Liberties Union (ACLU) executive director Anthony D. Romero in January 2002.

History shows that curtailment of civil liberties—including the right to free speech, the right to a fair trial, and the right to equal protection under the law— has often followed national...