We are in Iraq not just to defend the Iraqis, but to defend OUR OWN SECURITY. If we withdraw too early, the terrorists will take over Iraq and will institute a Taliban or Iran like terrorist state, from which to influence the Middle East and perpetrate terror attacks against us.
THEN we will see not 3000 casualties in Iraq, but 300,000 civilian casualties in the US, or more, when the terrorists will attack us with WMD.