KENNETH R. ROSEN joined the staff of The New York Times in 2014.

In the arid desert south of Tal Afar, members of Hash’d al-Sha’bi, which comprises nearly 60 predominately Shiite paramilitary units, have set up operations in this barren region outside of Mosul. Since last October, the group, also known as the Popular Mobilization Forces (PMF), has severed the Islamic State’s (ISIS’) supply routes to Syria using only their Jeeps, Hummers, and light tank weaponry, and in December, drove ISIS from the area.

When I arrived in early May, I witnessed a Jeep of militiamen tearing across the flat, dry expanse. As the truck streamed past the molehills of earth lining the various checkpoints, it churned up a cloud of dust. At the time, the Iraqi Army’s 9th Armored Division, the Federal Police, and the elite, U.S.-trained Rapid Response Units had surrounded the Old City of Mosul from all sides and the defeat of the militants seemed imminent.

Although this federation of fighters had largely remained dormant in the siege on the city, it fought many of the battles outside the Kurdistan region and was key to the victories of the Iraqi security forces against ISIS. In November, the Iraqi Parliament formally recognized Hash’d al-Sha’bi for its service, incorporating the militias into the Iraqi army through a bill supported by 208 of the parliament’s 327 members. But as the fighting winds down, the PMF is facing another rather complicated battle, this time over its legitimacy and its place in the country’s post-ISIS future. Since half of the units fought against U.S.-led coalition forces during the 2003 invasion of Iraq, the PMF has been dogged by allegations of war crimes and human rights violations.

In the most recent instance, according to a Human Rights Watch report first obtained by Foreign Affairs, one of the PMF brigades, the Liwa Ali al-Akhbar, may have arbitrarily detained and abused civilians over the course of interrogations meant to uncover whether the residents of newly liberated areas had ties to ISIS.