June 2, 2016 6:03 AM EDT

The Iraqi military and its allied militias are engaged in intense fighting on the edges of Fallujah in an effort to reclaim the city from ISIS militants. The offensive is a critical test for Iraq’s disparate armed forces in the broader war against ISIS, which seized a large portion of Iraq in 2014.


An estimated 50,000 civilians remain trapped in Fallujah, roughly 40 miles west of Baghdad. ISIS is losing territory in both Iraq and Syria, and the militants may attempt to impose a high human cost for any military victory by pro-government troops. Iraqi forces cut the supply lines into Fallujah in February, placing the city under siege and forcing thousands of trapped civilians to go hungry.


The Iraqi military is fighting alongside Shi’ite-majority militias called Popular Mobilization Units. Backed by Iran, the dominant Shi’ite power in the Middle East, the militias arose in 2014 in response to the collapse of the Iraqi national army in the face of ISIS. Critics worry that sending the Shi’ite militias into Sunni-majority Fallujah is a recipe for sectarian violence, even if ISIS is defeated.


Should pro-government forces expel ISIS from Fallujah, they will face the difficult task of earning the trust of members of Iraq’s Sunni Muslim minority, who have been skeptical of the central government in Baghdad in the years since the U.S. removed Saddam Hussein from power in 2003. Sunnis lost the relative dominance that they had enjoyed under Saddam, himself a Sunni, and subsequent Shi’ite-led Iraqi governments have failed to bring Sunnis back into the political process. Sunni alienation is one of the conditions that enabled ISIS–a Sunni-led group–to take control of Fallujah in the first place.


This appears in the June 13, 2016 issue of TIME.

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