As ISIS makes sweeping gains in Iraq, it is worrying to note that for the most part, the group is being fought by sectarian extremists.

Shiite militias – often acting as unofficial affiliates of Iran’s military – are operating outside of the Iraqi government’s tenuous reach. To some extent, these groups are working within Iraq’s popular mobilization forces (PMF), a nationally-sanctioned umbrella organization for the predominantly Shiite, and too often extremist, militias. The most powerful and prominent of these Iranian-backed militias are the Badr Organization, Asaib Ahl al-Haq, and Kata’ib Hezbollah. Their leaders are linked to a growing oeuvre of human rights violations, assassinations, and terrorist bombings. Their members have killed thousands of U.S. soldiers.

The Badr Organization is run by seasoned Shiite politician and leader of the PMF, Hadi al-Amiri, who has a history of instigating sectarian violence in Iraq. Between 2004 and 2006, al-Amiri reportedly ordered attacks on up to 2,000 Sunnis. According to a leaked cable from the U.S. State Department, “One of [al-Amiri’s] preferred methods of killing allegedly involved using a power drill to pierce the skulls of his adversaries.”

Today, areas where the Badr Organization fights ISIS have seen “some of the most high-profile Sunni-Shiite violence of the current conflict,” according to the Washington Post. Meanwhile, al-Amiri wields tremendous political and military power in Iraq, directing the country’s army and police in Diyala province, and even commanding the army’s 20th Battalion. The Badr Organization’s political branch holds 22 seats in the country’s parliament. Al-Amiri has himself been linked to a 1996 attack in Saudi Arabia that killed 19 U.S. Air Force servicemen.

Abu Mahdi al-Mohandes, also known as Jamal al-Ibrahimi, is the leader of Kata’ib Hezbollah (KH) and the deputy leader of the PMF. Al-Mohandes is also Iraq’s deputy national security advisor and a former member of the Iraqi parliament. He has been sentenced to death in absentia for his alleged involvement in the 1983 U.S. and French embassy bombings in Kuwait, attacks that killed six, including five Americans, and injured nearly 90 others. Al-Mohandes has also been linked to the 1985 assassination attempt of Kuwait’s Emir. He is designated as a terrorist by the United States.

Qais al-Khazali is the founder and leader of Asaib Ahl al-Haq (AAH). During the U.S.-led counterinsurgency, al-Khazali was one of the most wanted men in Iraq. In March 2007, al-Khazali was found and captured by coalition forces, but he was released in January 2010 as part of an apparent prisoner-hostage exchange. His group has claimed responsibility for more than 6,000 attacks on U.S. and Iraqi forces. Al-Khazali himself reportedly led the January 2007 AAH attack in Karbala that killed five U.S. soldiers.

Each of these groups has displayed a strong sense of loyalty to the Supreme Leader of Iran, Ayatollah Ali Khamenei, as well as Qasem Soleimani, head of Iran’s Quds Force, who reportedly coordinates military operations for all three militias. KH and AAH have explicitly rejected any cooperation with the United States in combatting ISIS. As of 2015, only KH and its leader Abu Mahdi al-Mohandes are designated by the United States.

As we seek to combat the brutality and horror of ISIS, we should keep in mind that the line of defense against ISIS is warped and untrustworthy, but necessary. Those forces keeping ISIS from Baghdad are armed not only with Iranian-backing, but with historical enmity towards the United States and non-Shiite Iraqis.

As Prime Minister Abadi attempts to build a more stable and at least nominally inclusive government, it has sought to bring these brutal figures into the political fold. But the fight against ISIS today is one of fire against fire. With these three militias leading the way, ruthlessness and sectarianism from ISIS is often being met with ruthlessness and sectarianism from the Shiite militias. Efforts by at least some of these militia leaders to repackage their forces as nationalistic and inclusive should be met with wariness at the very least.