Iraqi Shiite militias have officially joined the fight to retake Mosul from ISIS, despite calls from groups like Human Rights Watch to ban these abusive militias from participating in the fight for the last remaining major city in Iraq under ISIS control. Collectively known as Popular Mobilization Forces (PMF), many of these groups have deep ideological and financial ties to Iran and a long history of alleged human rights abuses against Sunni populations. Given their history of sectarian violence, these groups and their leaders will continue to prove challenging in any post-ISIS Iraq.

Here’s a closer look at the thorny history of the three main Iraqi Shiite militias:

Badr Organization

Origins: The Badr Organization is a Shiite political party and militia, and Iran’s oldest proxy in Iraq. Formed in 1983, members of the Badr group left Iraq to fight alongside Iranians during the 1980-1988 Iran-Iraq war. After the fall of the Hussein regime in 2003, the Badr group returned to Iraq and rebranded itself as a political party, pledging to abstain from further acts of violence.

Human Rights Abuses: Despite the pledge, the Badr Organization quickly garnered a reputation for carrying out brutal sectarian violence. From 2004 to 2006, the group’s leader, Hadi al-Amiri, is accused of ordering attacks on up to 2,000 Sunni Iraqis. Since 2014, the group has also been documented carrying out summary executions of Sunnis and widespread burning and demolishing of homes. One Human Rights Watch employee in 2015 said that out of all the militias fighting ISIS, “we’ve documented the most abuses… definitely [by the] Badr Organization.” Despite a long history of sectarian violence, Hadi al-Amiri—with deep ties to Iranian Quds commander Qasem Soleimani—has wielded tremendous influence in Iraq, having served a number of high level officials and in behind-the-scenes roles in the Iraqi government.

Kata’ib Hezbollah

Origins: Kata’ib Hezbollah is a U.S.-sanctioned Iraqi terrorist organization formed in 2006.  During the U.S. war in Iraq, Kata’ib Hezbollah earned a reputation for planting deadly roadside bombs and using improvised rocket-assisted mortars to attack U.S. and coalition forces. The group is responsible for some of the deadliest attacks against U.S. forces, according to U.S. diplomat Ali Khedery. Its leader, Jamal Jaafar Ibrahimi—also known by his alias Abu Mahdi al-Mohandes—is the alleged mastermind behind the U.S. and French embassy bombings in Kuwait in 1983 and the 1985 assassination attempt on Kuwait’s emir.

Human Rights Violations: After the battle to recapture Tikrit in 2015, Kata’ib Hezbollah was accused, alongside Badr and Asaib Ahl al-Haq, of responsibility for “the brutal aftermath to the fighting,” carrying out summary executions of Sunnis and “indiscriminate attacks in civilian areas,” according to a report by Human Rights Watch.

Asaib Ahl al-Haq

Origins: Formed in 2006 during the U.S. war in Iraq, Asaib Ahl al-Haq (“League of the Righteous”) carried out more than 6,000 bombing and kidnapping attacks targeting U.S. soldiers. Like the other two prominent Shiite militias, AAH is loyal to the Iranian regime, showing deference to the Guardianship of the Jurists (velayat-e faqih), the judicial system spearheaded by Iran’s supreme leader Ayatollah Ali Khamenei.

Human Rights Violations: In recent years, monitoring groups have documented numerous sectarian and homophobic attacks carried out by AAH, including the massacre of dozens of Sunni men in Iraqi towns. In May 2014, AAH members published a list of 24 “wanted” individuals, the vast majority of whom were accused of carrying out “homosexual acts.” Two months later, AAH members beheaded two teenagers believed to be gay, and threw their heads into the garbage. According to anecdotes provided by police, these types of attacks and acts of intimidation characterize the organization as a whole.

For more information, see CEP reports on the Badr Organization, Kata’ib Hezbollah, and Asaib Ahl al-Haq.