Shiite militias have embedded themselves within the structures of the Iraqi government, which has become far too reliant on their power to contemplate cracking down on them. Together, they have committed horrifying human rights abuses: In early June, Shiite militias along with Iraqi security forces reportedly executed around 255 prisoners, including children. An Amnesty International report from June detailed how Shiite militias regularly carried out extrajudicial summary executions, and reported that dozens of Sunni prisoners were killed in government buildings.

The militias also played a leading role in the liberation of the besieged Shiite Turkmen town of Amerli. Kata’ib Hezbollah, a U.S.-designated terrorist group and direct Iranian proxy, even used Iraqi government helicopters to deliver arms and other supplies during the battle. Just as IS has captured and used U.S.- supplied vehicles, U.S.-made M1A1 Abrams’ tanks provided to the Iraqi government have flown sectarian Shiite banners and supported Kata’ib Hezbollah operations. Those tanks are not alone: U.S.-made armored Humvees, which Kata’ib Hezbollah once targeted during the Iraq War with rocket-propelled grenades (when driven by Americans), have also been taken by the militia and used in operations.

Iran has led the way in developing Iraq’s Shiite militias. Since May 2013, Tehran has bolstered its network of new and old Iraqi proxy groups to provide a steady flow of fighters to Syria. Some of these Iraqi forces, who had been fighting on behalf of President Bashar al-Assad’s regime, redeployed back to Iraq and form the nucleus of newer militia groups which are currently fighting the Baghdad government’s Sunni enemies.