The US airstrike that killed Iranian general Qasem Soleimani also killed another key figure in Iran’s role in the Middle East: Abu Mahdi al-Muhandis, who had traveled to Baghdad International Airport to meet Soleimani. Deputy head of the Popular Mobilization Units, a mostly Shi’ite group of pro-Iranian paramilitaries in Iraq, Muhandis was an important key to Iran’s penetration of Iraq, a figure similar to what Hassan Nasrallah is to Lebanon.His death has gone less noticed because it was overshadowed by Soleimani’s, but it is no less important. Muhandis was important because for years he helped build up not only his own armed militia, Kataib Hezbollah (KH), but also transformed the PMU into an official force that can project power throughout Iraq and into Syria. The PMU is more than Hezbollah: It is a massive organization of 100,000 armed men, with access to another 200,000 men who have served in its ranks. Raised to fight ISIS, it became an official force of the government in 2017. It is trying to become Iraq’s version of the Islamic Revolutionary Guard Corps (IRGC). Soleimani and Muhandis were working together on this effort.Muhandis also sent cadres into Syria, where his men likely played a role in weapons trafficking to Hezbollah. He was an instrument of Iran’s policy, ready to carry out attacks on America or Saudi Arabia, as orders came down last year. The US detected such orders, killing Muhandis for his plans and role. His death has angered some in Iraq, but others are pleased because his militia and others like it have suppressed protests in the country.If and when revenge comes for the killing, it may be KH members who strike at the US. If so, they will have earned their spurs through being guided by Muhandis.Muhandis was a quiet and outwardly modest man who shied away from donning a uniform full of medals. He modeled himself on Soleimani, even down to his beard. The two men could have been members of the same family. They were from the same ideological family. Muhandis was the less important of the two, but he did not seek out interviews or cameras.Here is a look back at who he was and why he mattered.On December 29, the US carried out five airstrikes against Kataib Hezbollah (the Hezbollah Brigade), an Iraqi-based Shi’ite militia linked to Iran and accused of recent rocket attacks that killed a US contractor and wounded US soldiers. The brigade is one of the most important of the pro-Iranian militias in Iraq. It has an extensive role in the Middle East, linking it to Iran’s Islamic Revolutionary Guard Corps and operations in Syria. Formed between 2003 and 2007 by Muhandis, it is a creature of Iran’s IRGC. Muhandis has threatened Israel in the past and was closely linked to both IRGC Quds Force commander Soleimani and Lebanese Hezbollah’s Hassan Nasrallah.MUHANDIS WAS the leader of Kataib Hezbollah. An Iraqi, he was born in Basra and fled to Iran in the 1980s during Saddam Hussein’s crackdown on Shi’ites, where he signed on to fight with Iran’s IRGC. He became a close colleague of Soleimani and was linked to the military wing of Iraq’s Dawa Party at the same time, illustrating his influence among Iraqi Shi’ites in Iran.Michael Knights, an expert on Iraq, wrote in 2010 that Muhandis’s life “describes the arc of Iranian support for Iraqi Shi’ite proxies.” He returned to Iraq after the fall of Saddam and became a member of parliament, blending his militia activities with politics. KH was described in 2010 as a compact movement of only 400 personnel under the command of the Quds Force. Muhandis was wanted for terrorism in Kuwait and was also sanctioned by the US in 2009.The US Treasury found that Muhandis and his group threatened the peace and stability of Iraq. He had committed acts of violence. It said he was an adviser to Soleimani and the Quds Force. As such, Kataib Hezbollah was designated as a foreign terrorist organization. It was accused of receiving money from Iran through various European banks and using the money to finance attacks on Americans in the years before the US withdrawal from Iraq in 2011.In 2014 when ISIS invaded Iraq, the Shi’ite cleric Ali al-Sistani issued a fatwa to raise Shi’ite youth to fight the ISIS threat. Kataib Hezbollah became one of the groups within the Popular Mobilization Units (PMU) that emerged in 2014. Muhandis became the deputy commander of the PMU, illustrating his importance.KH created two brigades in the PMU, the 45th and 46th. It also has affiliates in the 47th brigade, numbering some 3,000 fighters. It operated principally through the 45th Brigade. KH continued to be involved in extrajudicial activities, such as allegedly kidnapping a Qatari royal hunting party in 2015. It received $700 million to release the hostages, according to Forbes.The organization also built its own parallel state structures, storing munition and holding prisoners. One facility south of Baghdad may hold up to 1,700 prisoners, The Washington Institute for Near East Policy reported. A RAND report said it is one of the driving forces for Iranian policy in Iraq.Muhandis was one of the key players behind Iraq’s offensive against the Kurdish Peshmerga in October 2017 after the autonomous Kurdistan region held an independence referendum. He lowered the Kurdish flag in Kirkuk. His forces also clashed with the Peshmerga.IN JUNE 2018, Kataib Hezbollah headquarters in Albukamal, Syria, which was helping direct Iranian policy to aid the Assad regime and traffic weapons for Iran, was hit with an airstrike. KH blamed the US and Israel for the airstrike. Kataib Hezbollah was allegedly behind a drone strike on Saudi Arabia on May 14, 2019, and an attack near the US Green Zone the same month.A KH vehicle was struck in a drone strike in August 2019 near the Iraq-Syria border. Muhandis threatened Israel with a response to the attack. In September, Muhandis said the PMU needs an air force to confront Israeli and American airstrikes.The role of KH in Syria is one of the key threats to Israel because Iranian-backed groups from Iraq help Iran move weapons to proxies in Syria and then to Hezbollah. Muhandis said his forces would help Hezbollah in a future war with Israel.“I am proud of my relations with Nasrallah,” he said in January 2017. “They go back a long time. He is the leader of the resistance and an important figure in the region. We follow his example and the example set by our brothers in Hezbollah.”Muhandis worked with Hezbollah on various issues, saying as part of the Iranian “resistance” he was working against the US and Zionism in the region.Kataib Hezbollah is not as large as Lebanese Hezbollah, but it plays a significant role in Iraq because of its direct links to Qasem Soleimani and the PMU. It has sought to transform the PMU into Iraq’s version of the IRGC. It has also extended its network into Syria to aid Iran’s movement of precision-guided munitions and weapons to Syria and to Hezbollah.The decision by the US to strike at KH was taken after intelligence showed KH’s role in rocket attacks going back more than six months. It was not a decision that was taken lightly: Muhandis had influence and the ability to strike back at a time of his and Iran’s choosing.The five strikes on both sides of the Iran-Syria border show the extent of the KH network. It has purposely colonized areas at Al-Qaim and Albukamal so that it can control the border crossing that is key to Iran’s “land bridge,” or road, to the sea. It may be central to Iran’s plans to move ballistic missiles to Iraq, revealed by Western intelligence services in August 2018 and December 2019.Now, KH is calling for a response to the US airstrikes. Iran is contemplating that response. KH’s long network from Beirut to Baghdad, including its ability to strike at Saudi Arabia and Israel, reveals its threat to the region and to US forces. As Kataib’s leader, Muhandis was one of the central Iranian keys to the region.