Judged solely on outcomes, the decade-long war on terrorism has been a rousing success. Al Qaeda has been chased out of Afghanistan and Iraq. Its leader and most of his lieutenants are either captured or dead. The organization is on the brink of “strategic defeat,” according to both the U.S. Defense Secretary and the director of the CIA. Across the Middle East, support for jihadist violence has plummeted. Since Sept. 11, 2001, al Qaeda has not mounted one successful strike on American territory—and has failed even to pull off an attack against U.S.