According to the United States these groups are funded, trained, and armed by the Iranian Quds Force, part of the Islamic Revolutionary Guard Corps (IRGC).

According to the United States Department of Defense, 603 American troops in total were confirmed to have been killed by IRGC-backed Shia militias (Special Groups) during the Iraq War.

After the US overthrow of Saddam Hussein, these Iranian-led militia men returned to Iraq where they retained their autonomy and Iran continued to support Shia Islamist paramilitaries.

The Iranians have "gone to a more sophisticated program with a smaller set of extremists" and are now focusing on three groups, which he identified as Kataib Hezbollah, Asaib Ahl al-Haq (League of the Righteous), and the Promised Day Brigade.

[1] Since the beginning of the Iraqi war against ISIS, the Special Groups have joined the Popular Mobilization Forces to fight against the Islamic State of Iraq and the Levant.