Respected military analyst Anthony Cordesman writes this in the New York Times:
The United States can win in Iraq only through offensive action. It cannot afford to make every American base a fortress, or to disperse scarce manpower and other military resources in force-protection missions. United States forces have to be mobile and able to redeploy where the threat is - even though such redeployments often mean moving forces to vulnerable areas. If the Pentagon concentrates on protecting troops in the short run, the war will last longer and total casualties will be greater. Worse, the United States will simply never win.
There is no certainty that the United States will win in Iraq. The war after the war is a far more difficult one than the war against Saddam Hussein. If America overreacts to attacks and lets the enemy drive its agenda, losing the war in Iraq will become not just possible but almost certain.