Here another source:

http://theweek.com/articles/589924/brief-history-isis


Where did they come from?

ISIS was born out of the U.S. invasion of Iraq in 2003. When U.S. administrators, under Paul Bremer, decided to "de-Baathify" the Iraqi civil and military services, hundreds of thousands of Sunnis formerly loyal to Saddam Hussein were left without a job — and they were mad. Al Qaeda chose to capitalize on their anger and established al Qaeda in Iraq (AQI) to wage an insurgency against U.S. troops in Iraq (Saddam was secular, but his intelligence and military supporters were able to make common cause with the jihadis of al Qaeda).