The reason that this attack occured is not rooted merely in the disastorous foreign policy of the US. The reactionaries that appear to have perprated this act are several notches lower on the totem pole of morality, which is exactly why Soviet imperialism (and that's what it was) in Afghanistan was justified.
Perhaps I should reformulate the question: how can the US deter future acts of horrific terrorism without exacting more casualties than any action would prevent? Complete capitulation would seem to invite more terrorism. Should we accept terrorism as the price for doing what's ultimately right?
-- Luke