Middle East Briefing: Who is battling Islamic State in Iraq?

CIA Director John Brennan said Sunday that Iran's involvement is complicating the US mission and may stir up sectarianism. Iraqi forces have surrounded Tikrit in preparation for a final push into the city. 

AP

Disparate groups have a common goal in defeating the self-proclaimed Islamic State, but sectarian strains remain. Iran, Kurds, Shiite militias, and even some Sunnis want to see the extremists defeated, and a major military operation continues in Tikrit, in northern Iraq. It's unclear whether the coalition can survive the battle and, more important, a possible victory.