On December 29, the US carried out five airstrikes against Kataib Hezbollah, an Iraqi-based Shi'ite militia that is linked to Iran and is accused of rocket attacks that killed a US contractor and wounded US soliders. Kataib Hezbollah is one of the most important of the pro-Iranian militias in Iraq and a group with 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 Abu Mahdi al-Muhandis it is a creature of Iran’s IRGC. Muhandis has threatened Israel in the past and is closely linked to both the IRGC Quds Force commander Qasem Soleimani and Lebanese Hezbollah's Hassan Nasrallah. Abu Mahdi Al-Muhandis is 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. He signed on to fight with Iran’s IRGC and became a close colleague of Qasem Soleimani, the head of the IRGC’s Quds Force. He 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 Knight, an expert on Iraq, wrote in 2010 that Muhandis’ 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. Kataib Hezbollah 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 also was 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 advisor to Soleimani and the Quds Force. As such Kataib Hezbollah was designated as a foreign terrorist organization. Kataib 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, abbreviated often as KH, 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 extra-judicial activities, such as allegedly kidnapping a Qatari royal hunting party in 2015. It received a large payment to release the hostages. According to Forbes it got $700 million. The organization also built its own parallel state structures, storing munition and holding prisoners. In one facility south of Baghdad may hold up to 1,700 prisoners, according to a piece at The Washington Institute for Near East Policy. A RAND report argues that 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 a Kataib Hezbollah headquarters in Albukamal, Syria 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 Kataib Hezbollah vehicle was struck in a drone strike in August 2019 near the Iraq-Syria border. Muhandis threatened Israel with a response to the attack. Muhandis said in September 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 where Iranian-backed groups from Iraq help Iran move weapons to proxies in Syria and then to Hezbollah. He has 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.” He has worked with Hezbollah on various issues. Muhandis argues that as part of the Iranian “resistance” he is 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. Muhandis is one of the central Iranian keys to the region.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 likely taken lightly because Muhandis has 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.