What do Islamist terrorists want? The answer should be obvious, but it is not.A generation ago, terrorists did make clear their wishes. Upon hijacking three airliners in September 1970, for example, the Popular Front for the Liberation of Palestine demanded, with success, the release of Arab terrorists imprisoned in Britain, Switzerland, and West Germany. Upon attacking the B'nai B'rith headquarters and two other Washington, D.C. buildings in 1977, a Hanafi Muslim group demanded the canceling of a feature movie, Mohammad, Messenger of God," $750 (as reimbursement for a fine), the turning over of the five men who had massacred the...