“ISIS (which ISIL is also referred to as) was able to infiltrate Iraqi government ministries and has the support of members of Saddam Hussein's Baath party, the Islamic Front (originally formed in 2004 as a resistance group to the U.S. occupation) and other Sunni groups,” Emran El-Badawi, director of the Arabic program at the University of Houston, said in an email.

The relationship between ISIL — an Al-Qaeda breakaway group — and other Sunni groups goes back to the beginning of the Iraq war. In the 2005 surge in Iraq, local tribes with U.S. funding built a coalition (known as the “Sunni Awakening Movement” or “Sons of Iraq”) that began combatting Al-Qaeda and other extreme groups to restore security and calm sectarianism. The program met with some success.

"These 90,000 ‘Sons of Iraq’ made a significant contribution to the reported 90% drop in sectarian violence in 2007-2008," said an op-ed co-written by Derek Harvey, a former senior intelligence official, and Michael Pregent, a former U.S. Army officer and onetime senior intelligence analyst.

But all that changed after U.S. forces withdrew and Maliki refused to integrate the Sunni tribes into the government. Off the payroll and pushed aside, the Sunnis were at a disadvantage and felt abandoned — while the Shias had full control of Baghdad and the south, and the Kurds had control of much of the north.

http://america.aljazeera.com/articles/2014/6/19/isil-thousands-fighters.html

The Sunni-Shia divide

The story of Islam's division between Sunni and Shia started with the Prophet Mohammed's death in 632.

There was a power struggle over who would succeed him in ruling the Islamic Caliphate, with most Muslims wanting to elect the next leader but some arguing that power should go by divine birthright to Mohammed's son-in-law, Ali.

That pro-Ali faction was known as the "Partisans of Ali," or "Shi'atu Ali" in Arabic, hence "Shia."

Ali's eventual ascension to the throne sparked a civil war, which he and his partisans lost.

The Shia held on to the idea that Ali was the rightful successor, and grew into an entirely separate branch of Islam.

Today about 10 to 15 percent of Muslims worldwide are Shia — they are the majority group in Iran and Iraq only — while most Muslims are Sunni.

"Sunni" roughly means "tradition."

Today, that religious division is again a political one as well: it's a struggle for regional influence between Shia political powers, led by Iran, versus Sunni political powers, led by Saudi Arabia.

This struggle looks an awful lot like a regional cold war, with proxy battles in Syria and elsewhere.