The Quds Force's roots stretch back to the Iran-Iraq war in 1980. Iran, which sought international influence amid heightened tensions in the Middle East, established the paramilitary group to conduct warfare through the use of proxies.
As the elite branch of the Islamic Revolutionary Guard Corps, the country's hardline military force that reports directly to the supreme leader, the Quds Force has been linked to violent plots and attacks against the US and its allies.
One method the Quds Force uses in destabilizing the Middle East is the use of military aid and funding. The group was discovered to have funded numerous organizations designated as terrorist groups by the US, including Hezbollah in Lebanon and Hamas in Gaza.
As a consequence of its actions, the Quds Force and the Republican Guard have also been designated as foreign terrorist organizations by the US.
In 2007, the US designated Qassem Soleimani, the Quds Force chief, as a terrorist and imposed sanctions against him, pointing toward roughly $100 million to $200 million that was provided to Hezbollah and weapons that were provided to the Taliban.
Within Iraq, the Quds Force trained militant groups that attacked the US through the use of sophisticated explosives.
One such explosive, the explosively-formed penetrator (EFP), was particularly devastating for US troops and coalition allies because its metal projectile shattered even armored vehicles. Roughly 861 US troops were wounded and 196 were killed by Iranian-made EFPs in the last decade, according to a Military Times report citing the US military.
The Quds Force is also believed to be operating beyond the Middle East, including the US. Soleimani and other Iranian officials were accused of orchestrating a plot to kill the Saudi ambassador in Washington D.C.
The Popular Mobilization Forces (PMF) is an Iraqi militia group founded in 2014 after Shiite leaders called for volunteers to stem the proliferation of ISIS in Iraq.
The group, which is composed of dozens of armed cells, were estimated to total around 122,000 fighters, according to Iraqi officials in 2018. The PMF was officially recognized by the Iraqi government in 2016, despite concerns from Sunni leaders who warned that it "looks like Iran's Revolutionary Guard."
While the founding principle of PMF's origins, the elimination of ISIS, remained aligned with the Iraqi government and US interests, the US warned that it would soon pose a threat as ISIS territory waned.
"Now, without a common enemy, these militias have no explicit purpose, yet most refuse to disband and relinquish control over areas they control," Philippe Atallah of the Foreign Policy Research Institute writes. "The future of these militias is unclear, and the Iraqi government needs to take control of them or risk losing authority to militia leaders who act as Iranian proxies and regional warlords with personal armies."
Many of these individual groups within the PMF have pledged loyalty to Iran and received direct support. After one of the Iran-backed PMF's brigades launched over 30 rockets at an Iraqi base, killing one American contractor and wounding four US troops, the US responded by conducting airstrikes against five bases, killing an estimated 25 fighters.
Violent protests erupted soon after the US airstrike, and the US embassy in Baghdad was mobbed by members of the PMF. Leaders from the organization were eventually blacklisted by the US for its role in the violent attack against the US embassy in Iraq.
The Badr Organization, or the Badr Brigade, is one of the largest militias within the PMF. The Iranian organization was originally formed by Shiite defectors in 1982 in response to the Iran-Iraq war. It later became its own political party.
Following the demise of Saddam Hussein, former members of the Badr Organization returned from exile and joined the Iraqi government and police ranks, where they fueled extreme cases of sectarian violence, including "using a power drill to pierce the skulls of ... adversaries," according to a State Department cable in 2009.
"The Badr Organization is deeply committed to Iran's Shiite revolutionary doctrine," the nonpartisan think-tank Foundation for Defense of Democracies said in a study.
The organization's chief, Hadi al-Amiri, claimed in 2015 that Iranian Supreme Leader Ali Khamenei "is the leader not only for Iranians but for the Islamic nation."
The organization, which remains close with the IRGC's Quds Force, was involved in skirmishes against US forces in the country during Operation Iraqi Freedom. Members have often publicized their use of US-built weaponry, including M1 Abrams tanks and the armored Mine-Resistant Ambush Protected vehicles.
Kataib Hezbollah, another Iran-backed PMF brigade, was formed by five smaller Shiite militant groups after the US's invasion of Iraq.
The organization embraces Iranian leaders and ideology, pledging to form a government similar to that of Tehran.
The brigade's leader and deputy PMF commander, Abu Mahdi al-Muhandis, was killed in the same airstrike that killed Quds Force commander Qassem Soleimani on Thursday. Muhandis previously described Soleimani as "a living martyr."
"I will not shy away from mentioning the support of the Islamic Republic of Iran in terms of weapons, advising, and planning," Muhandis said in 2018, according to Kurdish media.
In mid-October, Soleimani secretly met with militia commanders and directed Kataib Hezbollah to oversee a campaign of rocket attacks against US troops at Iraqi bases, according to a report Friday by Reuters.
The US State Department designated Kataib Hezbollah as a foreign terrorist organization in 2009. The group would continue to antagonize the US and threaten to fight "American occupiers" in Iraq.
Kataib Hezbollah agents killed five US soldiers during a rocket attack in Baghdad in 2011. According to the State Department, the group filmed their ambushes against US and Iraqi targets, including sniper and improvise-explosive device attacks.
Kataib Hezbollah's membership is estimated to be around 1,000 people, to the groups claim of 30,000 people.