One way of measuring any terrorist attack is to look at whether the killers accomplished everything they set out to. On Sept. 11, 2001, al-Qaida 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 9 that morning, who can doubt that they'd have maintained their pristine 100 percent success rate? Throughout the IRA's long war against the British Crown, two generations of politicians pointed out that there would always be the odd ''crack in the system'' through which the determined terrorist would slip. But on...