The question of how expansive the retaliation should be is difficult partly because there is only a grey area dividing those who are "responsible" and those who are not. The ones most directly responsible are the ones who hijacked the planes; obviously no retribution can be carried out on them since they're dead. Next level is the group of people, the ground support, architects, and financiers of the plan who now hide; I think international opinion would not be very controversial on how to deal with them. Next level out might be those who were not involved with this attack but are currently harboring plans to carry out similar attacks in the future, and this level out is probably where the controversy begins: is it moral to kill those who have not done anything wrong, yet? How, precisely, is "harboring plans" defined, and how much proof is needed to determine this?

Lowering the threshold of accountability might reduce the probability of future attack. At the same time, the larger the net cast, the more difficult and questionable the search, and thus the greater the potential for radicalization of currently law-abiding people (the vast majority, it should be noted).

Both those who urge all-out war and those taking the idealistic but naive position that we should "turn the other cheek" seem to me to be oversimplifying a very difficult problem. But I am happy to admit that I have no idea what the proper response is. If anything, I would urge people to consider reducing the certainty of whatever opinion you have, especially right now, when the general public has really no knowledge of detailed facts of the web of people responsible.

If the solution to terrorism were obvious, we would've done it long before now.