Iran's growing influence in Iraq is setting off alarm bells in the Pentagon, and nowhere is the problem starker than in the high-stakes battle for Tikrit.

It represents a crucial fight in the bigger war to expel the Islamic State group from Iraq. And yet, Iran and the Shiite militias it empowers -- not the U.S. -- are leading the charge.

This is a political as well as a military problem for the Obama administration, which is under heavy criticism for negotiating with Iran over limits on its nuclear program.

The battle for Tikrit raises the question: Who is running this war? Iraq? The U.S.? Iran?

Defense Secretary Ash Carter acknowledged in congressional testimony this week that he is alarmed by Iran's growing influence.