Al-Qaeda-linked Sunni militants are continuing their advance into Iraq today. A prominent Shiite cleric has issued a call to arms to fight the militants, raising serious concerns that the country is backsliding into sectarian conflict.

These Sunni militants are affiliated with a group known here in the U.S. as ISIS — or the Islamic State of Iraq and Syria.

We find out more about ISIS from Douglas Ollivant, senior national security fellow at the New America Foundation. He's also a retired Army officer who served in Iraq and the former director on Iraq for the National Security Council.