One way of measuring any terrorist attack is to look at whether the killers accomplished everything they set out to. On September 11, 2001, al-Qa'eda set out to hijack four planes and succeeded in seizing every one. Had the killers attempted to take another 30 jets between 7.30 and nine that morning, who can doubt that they'd have maintained their pristine 100 per cent success rate? Throughout the IRA's long war against us, two generations of British politicians pointed out that there would always be the odd "crack in the system" through which the determined terrorist would slip. But on...