Iran’s powerful military commander Qasem Soleimani was allegedly planning imminent attacks targeting Americans when a U.S. drone strike killed him Friday morning in Baghdad, according to U.S. officials. The Trump administration has not released any details about Soleimani’s alleged attack plans, though it has cited as evidence the major general’s recent trips across the Middle East, from Syria to Lebanon to Iraq.

Such travel, however, has been a mainstay of Soleimani’s work for decades as the leader of Iran’s Quds Force, a special squad focused on overseas operations, among other intelligence missions, as part of the elite Islamic Revolutionary Guard Corps (IRGC).

What is the Revolutionary Guard Corps?

When the 1979 Iranian revolution brought Ayatollah Ruhollah Khomeini to power and the Islamic Republic was established, Khomeini and his supporters had a problem: They weren’t sure they could trust the military, which just a short time beforehand had been aligned with the deposed shah. So they consolidated supporters and set up a parallel military force, called the Islamic Revolutionary Guard Corps, made up of skilled fighters that they knew were committed to guarding Iran’s new political system and the ideals of the Islamic revolution.

This two-tiered system is reflected throughout the bifurcated Iranian state: There’s the elected president, who oversees parliament and various ministries; then there’s the supreme leader, who is the nation’s highest political and religious authority and controls his own key political and financial institutions. The IRGC reports directly to the supreme leader, now Ayatollah Ali Khamenei.

Like Iraqi leader Saddam Hussein with the Republican Guard, the supreme leader uses the IRGC to enforce his will upon the Iranian people and squash any perceived threats, both from inside and outside the country.

On April 8, President Trump designated the IRGC as a terrorist group, the first time the United States had given that label to a foreign government entity. The designation enabled Washington to increase economic sanctions and political pressure on Tehran.

What is the Quds Force?

The Revolutionary Guard Corps is made up of several subgroups, including the Quds Force and the Basij militia. The latter are a paramilitary force mobilized to enforce order, including crushing protests such as the short-lived ones that swiftly swept through Iran in November. Like many related institutions in Iran, the Basij first formed as a volunteer force in the Iran-Iraq War and has since become an entrenched, and feared, part of the state.

Soleimani was the head of the Quds (or Jerusalem) Force, an ultra-elite group focused on overseas operations, such as developing and managing proxy militias, along with military intelligence work.

Soon after the Iranian Revolution, Iran and Iraq began a brutal eight-year war. The fighting helped the isolated and fledgling republic to consolidate power domestically but also took a very heavy toll on Iran’s society and economy — the effects of which are still felt today.

Iran consequently pushed forward with developing allies abroad, in an effort to spread its form of Islamic revolution, as well as to ensure it would never again be left alone on the battlefield. Through the Quds Force, Iran has cultivated Shiite militias in Iraq, Syria and Afghanistan, while also supporting groups such as Hezbollah in Lebanon and Hamas in the Gaza Strip. In the late 1990s, Soleimani was given command of the unit.

Following the 2003 invasion of Iraq, American forces along with their coalition allies found themselves facing Iranian militias backed by the Quds Force in the aftermath of the fall of Hussein’s regime. The Trump administration has blamed the deaths of over 600 coalition soldiers in Iraq on Soleimani and the pro-Iran militias he formed and directed. As head of the Quds Force, Soleimani was also a key architect of Syrian President Bashar al-Assad’s brutal attacks on anti-government protesters and rebels in the now nine-year war.

For a brief period, however, the United States and Soleimani’s proxies were working together in Iraq and Syria against the Islamic State. In 2014, Iraq set up the Popular Mobilization Forces (PMF), in Arabic known as the al-Hashd al-Shaabi, an umbrella organization of primarily Iranian-backed Shiite paramilitary groups united in the fight to oust the Islamic State. (The United States also killed one of the leaders of the PMF in the drone strike that killed Soleimani.)

Who is taking over for Soleimani?

Within hours of Soleimani’s death, Khamenei announced a successor: Ismail Qaani, the former deputy head of the Quds Force.

Soleimani and Qaani share some similarities: They both joined the force during the Iran-Iraq War, and the Treasury Department has imposed sanctions against both for alleged terrorist activities. Unlike Soleimani, however, Qaani lacks the cult of personality that surrounded his predecessor, who was both revered and reviled for having had a hand in so many of the attacks and wars that have shaped events and daily life in the Middle East in recent decades.

Read more: