This may be the worst news coming out of Iraq in a long time.

Thomas A. Ricks reports that "A battalion of the new Iraqi army refused to go to Fallujah earlier this week to support U.S. Marines battling for control of the city, senior U.S. Army officers here said, disclosing an incident that is casting new doubt on U.S. plans to transfer security matters to Iraqi forces.
It was the first time U.S. commanders had sought to involve the postwar Iraqi army in major combat operations, and the battalion’s refusal came as large parts of Iraqi security forces have stopped carrying out their duties." Some argue that it was a "command failure," others that it was just plain fear. There are other possibilities, as noted.

For a more academic analysis (based on iterviews of soldiers in Iraq) of why American soldiers fight, see this paper published in 2003: "Why They Fight: Combat Motivation in the Iraq War".

"The researchers then interviewed U.S. combat troops fresh from the fields of battle to examine their views. What they found was that today’s U.S. soldiers, much like soldiers of the past, fight for each other. Unit cohesion is alive and well in today’s Army. Yet, Dr. Wong and his fellow researchers also found that soldiers cited ideological reasons such as liberation, freedom, and democracy as important factors in combat motivation. Today’s soldiers trust each other, they trust their leaders, they trust the Army, and they also understand the moral dimensions of war." (Thanks to Phil Carter.)