Stupid
I doubt you'd know a rational though if it bit you. International terrorism is a different kind of war, but it is being fought. Going after those who carry out the attacks as well as those who support terrorists is making it much more difficult for them to strike. Capturing and interrogating terrorists has provided information on future plans, which makes those plans less likely to happen. Killing as many of those responsible also has the beneficial effect of taking them out of the picture.
On the other hand, looking for "root causes" and attempts at appeasement has a perfect track record of 0% success.