In Valentine Moghadam's chapter "Feminism On A World Scale," there is a statement from the Women's Center suggesting that it would be best if America, in response to the World Trade Center attacks, would "accept responsibility for the fallout from past foreign policies and ... refrain from military retaliation against Afghanistan..."
This has some validity in regards to America's past meddling in Afghanistan's political affairs during the Cold War. Also, violence in response to violence can't be the best approach to justice. However, if America were to just stand back and say "Yeah, we deserved that. Truce?" it would set a terrible precedent of there being no consequences for killing thousands of people other than some general international scorn. I'm not condoning war-waging, but it's the only way that people have been dealing with these issues for thousands of years, so it's very difficult to come up with and enact new methods. Also, the US military tries very hard to mitigate it's impact on innocent civilians via precise strike-groups and stringent intelligence collection. This at least centers the retaliatory violence toward specific, targeted people responsible for other acts of heinous violence. Again, war is a horrible institution that we must be able to do without somehow, but it's very deep-rooted in our civilization-- and murder should not go without consequences.