Prominent Iraqi Shia militia leader Aws al-Khafaji was arrested by men from the Popular Mobilization Units (PMU), the umbrella organization of the predominantly Shia armed groups. He was captured during a raid on his militia’s main office in Baghdad in early February. Several sources suggested that al-Khafaji was detained because of his recent criticism of Iran (al-Jazeera February 8).
Al-Khafaji is the leader of the Abu al-Fadhel al-Abbas Brigade (AFAB), a militia that he formed in Syria in 2012 to fight alongside the regime of Syrian President Bashar al-Assad under the pretext of protecting Shia shrines in that country. When Islamic State (IS) overran large parts of Iraq in June 2014, al-Khafaji extended his militias’ activities to his home country. At the height of its power, al-Khafaji’s AFAB was estimated to include 5,000 fighters. In the weeks that preceded his arrest, al-Khafaji became vocal in his criticism of Iranian dominance of Iraqi politics. He also accused his fellow Shia militia leaders of corruption. 
Aws al-Khafaji was born in 1973 in the city of al-Shatra in Thi Qar province of southern Iraq. He grew up in the city that, in the mid-20th century, was known to be one of the strongholds of the Iraqi Communist Party. The area, however, became more religious and conservative in the following decades. After graduating from the University of Basra with a degree in physical education in 1995, he chose to become a Shia cleric. He enrolled in Shia seminars (Hawza) in the holy city of Najaf, and quickly became a confidant of the late Shia cleric Muhammad Sadiq al-Sadr, the father of Moqtada al-Sadr and one of the most prominent senior clerics of his time. In the mid-1990s, al-Sadr the elder began building a network of regional representatives in the Iraqi provinces, and al-Khafaji’s Thi Qar was one of the areas where al-Sadr’s following was increasing. The network consisted mainly of young and enthusiastic clerics who became loyal disciples of Sadiq al-Sadr. Al-Khafaji was assigned by his leader to coordinate the most important activity of the network—the weekly Friday prayers which were being held across Shia areas in southern Iraq and the capital Baghdad. 
The Saddam Hussein government became more anxious about the growing influence of al-Sadr and his movement after years of tolerance. Al-Sadr was assassinated in February 1999 near Najaf. The Iraqi government then accused Iran and internal rivalries within the Shia clergy of being behind the assassination, but it was widely believed that the Iraqi Intelligence Service was behind the killing. Al-Kafaji had already become a suspect because of his perceived anti-government activities under al-Sadr’s leadership. He was arrested in early 1999, weeks before the killing of his mentor. He remained in jail for three years (al-Hayat, February 23, 2000). After the U.S.-led invasion of 2003, al-Khafaji assumed a senior role within the ranks of the followers of the Sadr family, led by its new patriarch, Moqtada.
Al-Khafaji became a prominent founding member of al-Sadr’s militia, the Mahdi Army. He was well known for his public incitement of hatred and call for attacks against the U.S.-led coalition forces. In 2007, he had to flee the country to avoid arrest. Though Iran became a preferred destination for al-Sadr and his followers, al-Khafaji chose to go to Syria. That might have been an early sign of al-Khafaji’s tendency to have a degree of insolence towards Iranian influence. While he was in Syria, then-Iraqi Prime Minister Nouri al-Maliki launched a crackdown against the Mahdi Army. Al-Sadr and al-Maliki have been political enemies ever since.
In 2010, Moqtada al-Sadr suspected that al-Khafaji was cooperating with al-Maliki, causing him to disavow al-Khafaji (Buratha News, November 11, 2010). Al-Khafaji returned to Iraq and worked under al-Maliki’s leadership. When he needed political cover and financial support for his militia, al-Maliki’s Dawa party provided it. But with al-Maliki out of office and the relative decline of the Dawa party, al-Khafaji grew weaker. His response was apparently to attempt to show more independence and promote moderate ideas.
Despite his history as a militia leader and a member of the Sadrist movement, al-Khafaji could not build his own base of support. He never had the status of his former leader, Moqtada al-Sadr, who inherited the Sadrist movement from his father. Unlike other Sadrist militia leaders like Qais al-Khazali or Akram al-Kaabi, al-Khafaji did not fully embrace Iran as a patron.
When he was detained there was no reaction from his militia. This was an example of the control the PMU leadership has over the militias it oversees, and which have mushroomed in size since 2014. The orders to arrest al-Khafaji was believed to have come from Abu Mahdi al-Muhandis, the deputy leader of the PMU and a longtime ally to Iran. The incident revealed that the PMU has its own security apparatus that is ready to move against the slightest dissent.
 See, for example, this clip from an interview published on YouTube on January 8: https://www.youtube.com/watch?v=dT1HUxXA5A0