One of the most important, but also least-understood, underpinnings of the turmoil that seems to be a permanent part of life in the Middle East is the tension between Sunni and Shi'a Islam. Below are a few articles that may be helpful in getting a grasp on some of the foundational issues.
First is a piece on the origins of the rift between Sunnis and Shi'a, as well as differences in the practices of the two versions of the Muslim faith:
Emjoy - I hope you find these helpful!
After posting the above, I came across a truly riveting documentary from Deborah Davies at Britain's independent Channel 4.
Death squads run rampant in Iraq's main cities, and in Baghdad, as many as 100 bodies a day are dumped on the streets. There are frequently signs of torture on the dead, but the killers have little to do with the either al-Qaeda or Sunni insurgents; they're Shi'a death squads. Davies investigates the links between these roving, organized murderers and high-ranking Shi'a politicians, and exposes how the militias these politicians control have systematically infiltrated and taken over entire police units and government ministeries.
It's a full television documentary program and clocks in at over 45 minutes, but it's well worth your time.