Abraham understood that the bottom line is conformity to God's will. God's commandment "Thou shalt not kill" assures us that those who mean to kill should be prevented. Sometimes the means and motivation of organized and determined killers are such that they can be prevented only by lethal force. This is regrettable, but insofar as it prevents the killings of more and more innocent others, not illicit.
Of course, Askel5 assures me war is acceptable provided it is waged by adults and not children.
To suppose one can abstain from warlike responses to acts of terrorism is to impose a paradigm of stasis on a dynamic world. The point of war is not vengeance for past injuries, but self-defense against prospective, opportunistic aggression. A communal option for comprehensive and dogmatic pacifism is indefensible because it inevitably condemns to death innocent others incapable of undertaking their own defense.