I believe we are showing the values and beliefs our country was founded on and lives by. If it were not true we would have sent enough nuclear missiles to make the middle east/ southwest asia one huge crater last Tuesday afternoon.
Our government has shown remarkable restraint where none was offered in return, untold amounts of mercy where none was given. That is not to say we forgive and forget when we can never expect the same in return.
The government is making sure of suspects, targets, intent, and purpose. We will undoubtably lose American servicemen in this fight only so as little civilian casualties as possible will happen (see above comment regarding nuclear missiles). Our citizens (and those of many nations who had people working in the WTC) were not afforded the same respect.