The U.S. military has concluded that the Sunni insurgency has significantly improved over the last few months. Officials and analysts assert that Saddam Hussein loyalists have succeeded in forming a command and control structure and improved their combat skills in the insurgency war against the U.S.-led coalition in Iraq. They said both Sunni and Shi'ite insurgents have also penetrated Iraq's military and security forces. "The enemy is effective," Defense Secretary Donald Rumsfeld said. "The enemy's got a brain. The enemy alters its tactics. As things happen on the ground, they see what we do to respond to it. They then change their tactic." The latest U.S. assessment was voiced in wake of the killing of at least 22 people in a U.S. military base in Mosul on Dec. 22. Officials said the attack was carried out by a suicide bomber who wore an Iraq Army uniform and might have fought the U.S. military in Faluja in November.