Anyway, moralist Germain Grisez came up with 6 principles that should guide a response to terrorism. He derived the list in September 2001. They are as relevant today as then. If only he had been listened to...
Anyway, here are his principles (emphasis mine):
1) Terrorism carries out an intention to kill or injure people and/or to destroy or damage things of value in order to instill fear as a motive for desired behavior. Instilling fear so as to motivate desired behavior often is counterproductive, but doing so sometimes is good and even necessary. Yet even if instilling fear is appropriate, terrorism is a morally unacceptable means, just because terrorists intend (though not as their ultimate objective or goal) precisely to kill, injure, destroy and damage.
2) People have at times put an end to isolated individuals´ acts of terrorism by killing them. However, terrorism carried out by members of a widespread group for ideological ends that appeal to extremists in that group presents a far greater challenge. Any possible response is likely to have only limited success at best. Yet when a community undergoing terroristic attack deliberates about how to respond, anger and hatred induce the illusion that very violent responses are likely --and perhaps almost certain -- to succeed.
3) The use of force to prevent terrorism can be justifiable and morally required of those responsible for defending the community. Even deadly force may be used against those one reasonably expects will otherwise continue to pose a grave threat. But force, especially deadly force, must never be used to avenge past acts or as terrorism to prevent terrorism. Such uses of force, even against military forces and assets, are morally unacceptable.
4) Moreover, when stopping terrorism requires the use of force against the activities of terrorists or of people complicit in their terrorism, any foreseeable damage to innocents (that is, people not engaged in those activities) must be no more than what those using the force would think it fair to accept if the innocents were their own friendly associates.
5) Responses to terrorism that are morally unjustifiable also are foolish. They provoke greater and more widespread anger and hatred: seven other demons will take the place of the first, and small atomic bombs will be used instead of hijacked airliners.
6) Even when carried out within proper limits, deadly force against persons cannot be an adequate response to terrorism. A sound response must also include a very serious and sincere effort to improve relationships with less radical members of the group whose interests the terrorists are trying to promote by their bad means. That serious effort at reconciliation must be implemented by economic and political action designed to mitigate suffering and reduce hatred.