Tensions between the US and Iran-supported Iraqi militias spilt into the open on Thursday when President Joe Biden ordered an attack on facilities in Syria belonging to Kataib Hezbollah and Kataib Sayyid Al Shuhada.
The US said the air strikes were in response to rocket attacks against an American military base in Erbil that killed a civilian contractor and wounded an American soldier.
The militias targeted are just two of many loosely grouped under Iraq's Popular Mobilisation Units, also known as Hashed Al Shaabi, which is nominally integrated into the Iraqi state and has the loyalty of a large proportion of the Iraqi parliament.
The US carried out similar reprisals in the past against Kataib Hezbollah, which played a crucial role for Tehran as forward troops of Iran's Revolutionary Guard Corps in a strategic region of Syria.
The US strikes on Thursday targeted positions near Al Bukamal, an Iraq-Syria border crossing used by Kataib Hezbollah and Kataib Sayyid Al Shuhada to transport weapons and goods.
It is an important link in the supply line from Iran to Lebanon’s Hezbollah movement, whose organisation and guerrilla tactics Kataib Hezbollah has copied.
Who are Kataib Hezbollah?
Kataib Hezbollah traces its origins firmly to Iran, and its presence in eastern Syria indicates Tehran's high level of trust in the group.
With other Iraqi militia and Shiite militants from Lebanon and Afghanistan, Kataib Hezbollah highlighted the protection of Shiite shrines and fighting ISIS as the main reasons for their presence in Syria.
But they focused on helping President Bashar Al Assad put down the revolt against his rule, contributing significantly to the regime’s siege warfare and the depopulation and forced transfer of inhabitants from Sunni rebel areas.
Kataib Hezbollah's former commander, Jamal Ibrahimi, better known by the nom de guerre Abu Mahdi Al Muhandis, launched the group in the mid-2000s.
He was killed alongside Iranian general Qassem Suleimani in a US air strike in Baghdad on January 3, 2020.
After the death of Al Muhandis, a new leader took charge of the group, Ahmad Al Hamidawi.
Before founding Kataib Hezbollah, Al Muhandis had served since the 1980s as a militia operative closely linked with Suleimani, who led the Revolutionary Guard's Quds Force, overseeing Iran's foreign militia clients.
Al Muhandis formerly led the Badr Corps, the military arm of the Supreme Council for Islamic Revolution in Iraq, which was founded in Tehran in the 1980s as an opposition group against Saddam Hussein.
He was one of several members of the Supreme Council who sought independence from the Al Hakims, the Iraqi political-religious family from Najaf that dominates the group, and branched out with Iranian support.
According to the Wilson Centre, a research institute in Washington, Al Muhandis had Iranian citizenship. He was convicted and sentenced to death in Kuwait for the 1983 bombings of the US and French embassies there but escaped the country.
In a report on pro-Iran militia in Iraq, the centre said that “from his years in Iran, Al Muhandis developed a close relationship with Suleimani, who once referred to him on Iranian television as a dear brother and ‘living martyr’”.
The US classified Kataib Hezbollah as a foreign terrorist organisation in 2009. The State Department said at the time that it is a “radical Shiite Islamist group with an anti-western establishment and jihadist ideology that has conducted attacks against Iraqi, US, and Coalition targets in Iraq”.
In early 2020, the US added Al Muhandis's successor, Al Hamidawi, to the specially designated global terrorist list.
Al Hamidawi joined Kataib Hezbollah in 2007, shortly after its founding and quickly reached the rank of commander.
He is believed to have played a central role in planning attacks against American and Iraqi government security forces between 2007 and 2011, before he left for Syria to support Mr Al Assad's forces.
Another Iraqi shiite militia group linked to Iran, Asaib Ahl Al Haq, was also designated as a foreign terrorist organisation in early 2020.
Mike Pompeo, who was US secretary of state at the time, called the Asaib Ahl Al-Haq militia and its leader Qais Al Khazali, "violent proxies of the Islamic Republic of Iran".
The group was also linked to the February 15 attack in Erbil that prompted Mr Biden's military response.
For years Kataib Hezbollah and other Iranian proxy militias attacked US forces in Iraq and formed death squads who massacred Sunni civilians while the US was focused on crushing a Sunni insurgency that became dominated by Al Qaeda and later ISIS.
The attacks by Iran-backed militias, which killed and maimed hundreds of American soldiers in Iraq, were somewhat forgotten in Washington as the Barack Obama administration sought a nuclear deal with Tehran in isolation from Iranian actions in the region.
Now, as Mr Biden looks to revive the nuclear deal abandoned by former president Donald Trump, many of the same challenges remain as critics wonder how he will negotiate with a government using proxy forces to attack American interests.
Mr Biden is walking a fine line between punishing Iran for its military aggression and remaining on terms good enough to persuade its leaders to resume compliance with the nuclear accord.
A version of this article was first published on December 31, 2019.