On this day eighteen years ago, al-Qaeda carried out the most ambitious and deadly terrorist attack in history. The Islamist group hijacked four large commercial jets and crashed three of them into major U.S. targets: the two towers of the World Trade Center in New York and the Pentagon in Washington DC. (Courageous passengers stormed the cockpit of the fourth hijacked plane, blocking its planned strike but leading to a crash in rural Pennsylvania, with all aboard lost.) The two towers fell, the Pentagon was partly wrecked, almost 3,000 people died, and Americans were terrified. The attack’s immediate success probably surprised even its planners. So what did 9/11 achieve for al-Qaeda? Nothing. In fact, the attack led to disaster for the Islamist group. That’s because terrorism does not work.