Time Magazine - Making Tribal War Work for the U.S. in Iraq. Excerpt follows:

"...The Iraqis fighting alongside the Marines in Huseybah may have been familiar to both the U.S. forces and the insurgents. Many members of the Iraqi Army's Desert Protectors unit are from the al-Mahal tribe in the al-Qaim area. These men had fought against the Marines of 2/1 last year when U.S. forces first moved into the region. But after the Marines had left, the al-Mahal lost a tribal dustup with the Karabilah tribe, which had allied with Abu Musab al-Zarqawi's Al Qaeda in Iraq group. So, the al-Mahal were kicked out of Huseybah and the jihadis moved in. Now, the al-Mahal are back helping their erstwhile enemies, the Marines, drive out their tribal foe—most of the fighters resisting Operation Steel Curtain are presumed to be from the Karabilah tribe. Ubaidi, about nine miles east of Huseybah, is considered a stronghold of foreigners..."