On December 2, 2001, al Qaeda second-in-command Ayman al-Zawahiri wrote, in Knights Under the Prophet’s Banner: “…the Islamic jihad movement must escalate its strikes and means of resisting its enemies, to keep up with the extraordinary increase in their number, the quality of their weapons, their destructive capacity, and their disregard for all taboos, even for the rules of engagement. In this regard, we must:
1. be sure to inflict maximum casualties on the enemy, kill the greatest number of people, for this is the language understood by the West, no matter how much time and effort such operations take;
2. concentrate on martyrdom operations as the most successful way of inflicting damage on the opponent and the least costly to the mujahedeen in terms of casualties;
3. choose our targets and weapons to reach the vulnerable spots of the enemy and thus dissuade it from its brutal ways, arrogance, and violations of every sacred custom and restore it to its normal place;
4. reemphasize that at this stage focusing on the domestic enemy alone would be pointless.”
– Gilles Kepel and Jean-Pierre Milelli, eds., Al Qaeda In Its Own Words, Pages 203-204