Then-Maj. Michael Zacchea in Fallujah, Iraq, late 2004.

Slowly I began to discern the divisions that separated the Iraqis from each other. The most obvious was rank. The officers strictly separated themselves from the enlisted men, to a degree much more stringent than we knew in the Marine Corps. Every expression, every gesture reinforced their separateness and their superiority. Their attitude toward the NCOs — those sergeants who acted as intermediaries between the officers and the enlisted Iraqis — was only slightly less condescending.

The second-most-obvious partition among the Iraqis was ethnicity. The Arabs considered the Kurds' loyalty suspect. The Kurds considered the Arabs' motives dangerous. The Arabs spoke Arabic, and the Kurds spoke two dialects of Kurdish. The couple dozen [soldiers in the battalion I advised] from other ethnic groups — Assyrians, Turkmen, and the like — clung to each other for safety.

The toughest schism to detect — but probably the most dangerous — was the religious divide. The Sunni-versus-Shiite division of twenty-first-century Iraq made me think of the Catholic-versus-Protestant division of sixteenth-century Europe. The Sunnis and the Shiites split apart almost at the very beginning of Islam, in the seventh century. ... Fourteen centuries later, the dissension was still just as real, right in front of me.

The fact that the Kurds were mostly Sunnis — and so had that in common with half the battalion's Arabs — scrambled things up that much more. We advisers had been taught about none of these rifts before we left the States. Probably nobody in the military knew to teach them to us. We learned on the fly.

Making things even more turbulent were a few basic facts about Iraq and our war. Most Iraqis are Shiites. But Saddam Hussein and his old power structure were Sunni. Both the Kurds and the Shiite Arabs resented the Sunnis in a deadly serious way. Then there were our enemies, the insurgents. Most of them were Sunnis. But in the south and around Baghdad, they were usually Shiites who were allied with Iran. The two insurgent sects also were fighting each other. In some places, subsects were fighting each other. The Kurds were loyal to the Americans, but their Peshmerga warriors were fighting insurgents in the north without consulting with us.

That said, the Iraqis' [animosity toward each other] seemed to evaporate when they dealt with each other one-on-one. Like people everywhere, in the abstract, their words were a lot tougher and meaner than their face-to-face behavior. A Kurd may casually refer to Arabs as "rabbits," because he thinks they're good for hunting down, but then he'll be friends with an individual Arab. A Sunni may like saying that Shiites are wannabe Iranians, but then he'll greet a Shiite acquaintance with a kiss on both cheeks. [My close friend and Iraqi officer] Major Zayn, for example, was a Sunni, but he shared a room and was close with two Shiite officers. ...

I stayed up with the three of them late nights, taking in everything I could. We sat on their scratchy wool blankets and talked about religion and the war and America. … The Iraqis loved talking about politics and religion. They went on and on about those two topics more freely with the Americans than we Americans are willing to do with each other. The Iraqis were intensely curious about the United States. So we talked, long into the night. Outside, it was 110 degrees. Inside, they turned the AC down as low as it would go. My teeth chattered, and they laughed at me.

I needed to learn as much as I could. I felt rushed to learn. Military units need cohesion. They do not need baked-in discord.

If we were going to overcome the Fifth Battalion's many partitions, we would have to do it on two levels: On a high level, we had to make them loyal to the concept of a unified Iraq where everyone is equal under the law. In other words, we had to make them Western. (Nobody back in the States had told us how to do that, either.) On a basic level, we had to work man by man, creating something cohesive between individuals that could supersede the cultural schisms. That meant building relationships between Americans and Iraqis, and building trust among the Iraqis themselves.

We had to find the best, most committed leaders who could hold the battalion together. The ancient schisms weren't going away.