If you're like me you might be scratching your head wondering why U.S. troops, specifically the Marines, haven't descended on Fallujah yet. Steven Den Beste provides one of the better explanations I've read to date:
If there are broad reprisals against the uncommitted friendly population because of the acts of the terrorists, that population will become motivated and polarized in favor of the position held by the terrorists. If the American response is viewed by the Sunnis as being directed broadly at all Sunnis, rather than being targeted specifically at those responsible for this outrage, then there's every likelihood that the Sunnis will begin to wonder whether the US is actually genuine in its attempts to include the Sunnis as equal partners in the new government of Iraq. That would be a major victory for al Qaeda.
This terrorist attack was an application of violence intended to derail the American effort to set up a liberal democracy in Iraq, by attempting to provoke an American reprisal which would lead to Sunni suspicion and reduce Sunni participation in that democracy. [...]What is needed is a response which simultaneously punishes al Qaeda and reassures the Sunnis. But to do that, there has to be preparation. Our intelligence people now are busting their butts trying to learn everything they can about this attack and those responsible for it. Until they begin to make headway in that process, we must wait.
He also points to an entry at Belmont Club, which indicates that the Marines have already quietly begun to respond. Good.