As Iraqi and Iranian forces fight side by side to retake the city of Tikrit from ISIS fighters, the author of Inside ISIS sat down with Brian Kilmeade to discuss what's going on. 

Iran's increasing involvement in Iraq is raising alarm among U.S. allies in the region, like Saudi Arabia, UAE, and Jordan.

The head of Iran's elite Quds Force, Gen. Qasem Soleimani, is reportedly on the front lines in Iraq, leading Shia militias in the fight against ISIS.

Author Benjamin Hall said this morning that this is a sign that Iran wants to take back not just Tikrit, but the "whole of Iraq."

Hall said in order to bring Turkey into the fight against ISIS, the United States must step up its efforts to oust Bashar Assad in Syria. 

"That's always been number one on Turkey's agenda," said Hall, adding that Turkey can help close its borders with Iraq and Syria and possesses the second-largest NATO military.

Since Hall reported from Iraq during the successful U.S. troop surge, he says the country has gotten "worse than ever" in the last six years. 

He said Obama pushed the Iraqi government toward an alliance with Iran by failing to agree on a long-term security agreement to maintain U.S. troops in Iraq.

Watch the interview above.

As ISIS cuts a swath across the Middle East with unparalleled brutality, how will the world respond. And will America lead, or follow? "Fox News Reporting" shows you the damage wrought by ISIS as few have seen it, with exclusive stories and footage of the devastated families whose only mistake was being in the wrong place at the wrong time.  And, Bret Baier exclusively speaks to President Abdel Fattah el-Sisi of Egypt, a rare leader in the region who has stood up to Islamic terror, and dared call it by its name. All this and more in "Unholy War - The March Of ISIS."

Don't miss this "Fox News Reporting" special Friday at 10p/1a ET, Saturday at 10p ET and Sunday at 8p ET on Fox News Channel.