With Iran in leading role in battle for Tikrit and US sidelined, where will it lead?

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.