The crisis in Iraq is largely a war between the two main branches of Islam-- the Sunnis and the Shia -- which split nearly hundreds of years ago, roughly similar to how Catholics and Protestants divided Christianity.
The governments of Syria and Iraq are largely Shia. The attacking force is Sunni. That force calls itself the Islamic State of Iraq and Syria or ISIS.
- What is going on in Iraq and why?
- Al Qaeda cuts off ISIS, affiliated rebel group in Syria, picking sides with rival al Nusra faction
- Iraqi al Qaeda and Syria militants announce merger
Along with territory, the ISIS fighters took prisoners as they swept through northern Iraq this week.
The group has an especially brutal reputation.
Online pictures appear to show the bodies of the Iraqi police and soldiers they murdered along the way.
In an online message on Thursday, the ISIS spokesman warned its fighters would march onward to Baghdad, and beyond to the Muslim holy cities of Kerbala and Najaf.
For 10 years, ISIS has had ambitions to set up a vast Islamic state.
Originally known as al Qaeda in Iraq, its fighters often battled U.S. troops during the American occupation.
Fast forward to 2011 and the start of Syria's civil war next door.
By then ISIS had a new name, and a new leader - Abu Bakr al Baghdadi.
Its fighters moved back and forth over the Iraq-Syria border, exploiting the chaos and brutality to become one of the most powerful groups on the battlefield.
In areas of Syria where they ruled, there were public executions and a code of strict Islamic law.
Meanwhile, the group is carrying out almost daily suicide bombings in Baghdad.
No one knows how many ISIS fighters there are, but they come from across the Arab world to join up with a group that's well funded with revenue from oil smuggling and extortion and steadily adding to its arsenal with weapons looted after every victory.
Many Islamic militants believe ISIS is now setting the extremist agenda, no longer al Qaeda, and they see the ISIS leader, al Baghdadi, as the new bin Laden.