On Monday, Iraqi security forces aided by Shiite militias started an offensive to take back the city of Tikrit, which has been under Islamic State control since last summer. This is at least the third major attempt to dislodge ISIS from the city—previous operations in June and August of last year failed.

Strategically, the battle for Tikrit is a crucial military challenge for an Iraqi army trying to wrest back control of the northern and western parts of the country that it lost to ISIS in the past year. As my colleague Steve Clemons noted on Twitter, the Tikrit operation is, for one thing, "an important training run on the larger eventual challenge of Mosul," the second-largest city in Iraq, which is also under Islamic State control.

But, perhaps even more importantly, the fight is a test of how the country's sectarian divide will play out in the fight against ISIS. Tikrit is not only the hometown of former Iraqi dictator Saddam Hussein, it is also dominated by Iraqi Sunnis who are wary of Baghdad's Shiite-led government and Iran's growing influence in the country.

Ahead of Monday's operation, Iraqi Prime Minister Haider al-Abadi made one last promise of amnesty for Sunni fighters in the city who have joined forces with ISIS. “I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities,” Abadi said on Sunday. Whether Sunni fighters (or civilians for that matter) in Tikrit will side with government forces remains a huge unknown.

Previous reports have noted that Iraq's Shiite militias, some of them with Iranian training and leadership, have recently been effective in fighting ISIS. But those militias have also committed serious abuses against Sunni civilians. And the country is not long removed from a period of civil war in which Sunnis and Shiites committed atrocities against one another. The New York Times noted U.S. warnings to Iraq not to send Shiite forces into Sunni areas—Iraqi officials told the paper that "among the nearly 30,000 fighters involved in the Tikrit operation were an estimated 700 to 1,000 Sunni tribal fighters."

This is the first effort to retake Tikrit since Abadi took over for the much-maligned Nouri al-Maliki, who marginalized the country's Sunnis as Iraq's prime minister. The offensive is also different from the previous efforts in that momentum no longer appears to be on ISIS's side. Aided by American-led airstrikes, Iraqi forces recently retook Beiji, the nearby refinery town, which like Tikrit also sits on the road to Mosul.

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