CNN  — 

The United States put Iran’s Islamic Revolutionary Guard Corps on its list of terrorist groups on Monday. The unprecedented move against another country’s armed forces is bound to rachet up tensions with Tehran. Here’s what you need to know.

What is the Revolutionary Guard and what does it do?

The IRGC is an elite wing of the Iranian military that was established in the aftermath of the country’s revolution in 1979.

The unit is at the forefront of Iran’s military operations in the region – it has a significant presence in Iraq and Syria, where its troops and advisors have played a major role in the fight against ISIS, and have sometimes been within miles of US military positions.

There are more than 150,000 troops in the IRGC, according to the Council of Foreign Relations. These include ground forces, naval and air forces, as well as intelligence and special forces. Their primary role is internal security but experts say that the group – considered Iran’s dominant military force – assist Iran’s regular army, which has roughly 350,000 soldiers, with external defense.

In 2009, the Guards’ Basij branch – one of five IRGC subgroups– cracked down hard on anti-government demonstrations in the wake of a contested election, helping to secure the position of hardline then-President Mahmoud Ahmadinejad.

The Revolutionary Guard chief, Major General Mohammad Ali Jafari, reports to directly to the country’s Supreme Leader Ali Khamenei. The IRGC is not only Iran’s most powerful military institution, it holds deep influence over domestic politics and the economy, with interests extending to and beyond the construction, telecommunications, auto and energy industries

The IRGC and the United States

The Revolutionary Guards have long been in the crosshairs of successive US administrations.

In 2007, the US Treasury Department designated the Quds Force – a Revolutionary Guards unit in charge of foreign operations – as a supporter of “terrorism,” describing it as Iran’s “primary arm for executing its policy of supporting terrorist and insurgent groups.”

After US President Donald Trump took office, he withdrew from the Iran nuclear deal and set about clamping down on Tehran’s influence in the Middle East, re-imposing a strict sanctions regime that was designed in part to curb the influence of the Guards.

US allies in the region also consider the Revolutionary Guards a major security threat, and Bahrain and Saudi Arabia added the IRGC to their terror lists last year. Israeli Prime Minister Benjamin Netanyahu has frequently railed about the Guard’s military activity in the region.