Since February 28, the US and Israel’s war on Iran has triggered several unintended spillovers in West Asia. Chief among these is in neighbouring Iraq, where an umbrella group of Iran-backed militias — the Hashd ash-Sha’bi or Popular Mobilisation Forces (PMF) — has been striking US bases in Iraq in support of Iran’s response.These attacks continue amidst a phased withdrawal of US forces under way in the country since January. On March 20, the North Atlantic Treaty Organization (NATO) separately announced the full withdrawal of its own forces from Iraq — 22 years after it formally joined the US-led military mission in Iraq, after the 2003 US invasion led to the removal of Iraqi president Saddam Hussein.But on March 23, US-Israeli airstrikes hit PMF’s provincial headquarters in Anbar, killing its operations commander and other personnel. This prompted the Iraqi National Security Council, chaired by Prime Minister Mohammed Shia al-Sudani, to authorise the PMF to respond to the US-Israeli attack.These developments reflect tectonic shifts in Iraq and West Asia, upending decades-long geopolitical equations.By June 2014, the sectarian Salafi-jihadist group, Islamic State of Iraq and Syria (ISIS), had taken over one-third of Iraqi territory, including Iraq’s second largest city Mosul. In response, Ayatollah Ali Sistani, Iraq’s highest Shia spiritual leader, issued a fatwa calling on all able-bodied individuals to take up arms to fight ISIS.This “popular mobilisation” spawned PMF, a militia far superior to Iraq’s formal army that was crumbling in the face of the ISIS onslaught. Come 2019, PMF had incurred high costs but was successful in retaking ISIS-controlled territory and re-establishing Iraqi sovereignty, helped by both US and Iranian anti-ISIS military operations.With ISIS largely defeated, PMF faced a crisis of purpose despite widespread popularity. It largely split into three informal factions with different loyalties: to Iran’s Islamic Revolutionary Guard Corps (IRGC), particularly its Quds Force spearheaded by General Qasem Soleimani; to Sistani’s more nationalistic — rather than religious — principles; and to Moqtada Al-Sadr, a powerful Iraqi cleric and fierce Iraqi nationalist.Story continues below this adTogether, they created a classic problem of disarmament, demobilisation, and reintegration for the Iraqi State, which sought to re-establish its monopoly on legitimate violence — a problem that other States housing Iran-backed militias also faced.Also in Explained | GDP: From the US to India, who are the economic winners and losers of US-Iran war?In 2019-20, Iraq announced that PMF would be formally incorporated into the Iraqi military, advancing a 2016 law that recognised them as “independent military formations” within the larger armed forces. This bypassed the seemingly impossible task of disarming them.The years since then have featured a protracted struggle within the Iraqi government, which was formally a US ally since 2003 and has maintained strong ties with Iran, whose IRGC was by then well integrated with PMF’s pro-Iran factions. And PMF’s actions became a microcosm of this struggle, as Tehran steered the pro-Iran units into a twofold role: to dominate Iraq’s formal elected institutions through political wings, and to militarily threaten US bases in Iraq and encourage their withdrawal.By 2021, these pro-Iran factions had become unpopular enough to trigger massive anti-Iran demonstrations across Iraq, which PMF helped to violently suppress.Iraq’s balancing actStory continues below this adFollowing two watershed events — the 2003 US invasion and the 2014-19 fight against ISIS — Iraq became a prominent geopolitical stage for US-Iran rivalry. After 2003, Iran had successfully leveraged Washington’s infamous disconnect with Iraq’s ground reality to expand its influence there. This caused consternation amongst Iraqi nationalists including Sistani, who has consistently preferred separation of Church and State unlike Iran.As a result, the pro-Sistani PMF units worked with the Iraqi defence ministry on anti-ISIS operations and restricted operations to within Iraq’s borders, whereas the pro-Iran factions coordinated with the IRGC-Quds Force and focused on operations against US bases both in Iraq and the broader region. Members from the Popular Mobilisation Forces attend a funeral of their colleague who was killed on March 23 airstrikes on Anbar, at Sadr city in Baghdad on March 28, 2026. Photo: AP/Hadi MizbanCrucially, the latter group contained three of the most powerful PMF groups: Kata’ib Hezbollah, Asa’ib Ahl al-Haq, and Kata’ib Sayyid ush-Shuhada. These units concentrated on resisting US military presence across Iraq, particularly Ain al-Asad (the largest US-operated Iraqi airbase), Camp Victory near Baghdad, the Baghdad Green Zone, and other bases. But internal Iraqi resistance to what PMF had become meant that for Iran, Iraq was still a contested geopolitical landscape where it was superior to the US but not dominant.This changed in early 2020, when the first Trump administration assassinated both Soleimani as well as a pro-Iran PMF heavyweight, Abu Mahdi al-Muhandis. For perspective on what they meant to Tehran, Iran named new cruise missiles and naval corvettes after both figures.Story continues below this adThe first Trump administration, which brought back an isolationist view, was also committed to withdraw US forces from Iraq — in line with Tehran’s objectives. But the two assassinations also tilted PMF’s balance internally, strengthening the pro-Sistani factions. Iraq’s nationalist politicians sought to use the new reality to push for more stability and reduce factionalism within parliament and the Iraqi military, including PMF.That PMF as a whole was largely kept in check, despite Israel’s war on Gaza since 2023 and the fall of the Iran-allied Bashar al-Assad regime in neighbouring Syria in 2024, was proof that it was being forced to act more pragmatically. Arguably, PMF was the only Iran-backed armed group which did not engage US-Israeli targets with the intensity that Hezbollah in Lebanon or the Houthis in Yemen did.More in Explained | US considers ground troops in Iran: The options before Trump, and what they say about the warIn any case, the second Trump administration’s redoubled focus on US withdrawal led to the US vacating Ain al-Asad completely by January 2026. The Iraqi leadership, having gone through a tumultuous election in November 2025, had a real chance to focus on reducing Iranian influence since the US withdrawal removed Tehran’s reasoning for its own deep-rooted presence. But this chance was demolished by February 2026.The present and futureAs the US and Israel began strikes on Iran and assassinated its Supreme Leader Ayatollah Ali Khamenei, pro-Iran PMF units benefitted from the pro-Iran sympathy wave that swept the streets in several Arab States. This included pro-Iran demonstrations in Iraq’s Basra province: interestingly, about 30,000 people there had protested Iran’s influence in Iraqi resource-sharing arrangements back in 2018, and had even set the Iranian consulate on fire.Story continues below this adOn March 9, Sistani issued another fatwa, this time calling the fight against US-Israeli forces a “collective religious duty”. Combined with the optics of the NATO withdrawal and the promise of a complete US withdrawal by September 2026 (including from Iraqi Kurdistan where US presence is now concentrated), pro-Iran factions foresee a chance for fresh dominance in Iraq.Amidst this surge in pro-Iran sentiments, the March 23 US strike against PMF left the Iraqi government with no choice but to support anti-US action. Iranian leverage over Iraq has also been strengthened due to the closure of the Strait of Hormuz, with supplies of Basra crude facing a 70% cut. Iran’s act of allowing some Iraqi ships to pass also presents the opportunity to enhance its goodwill in the eyes of Iraqis.Moreover, Iran would likely feel vindicated by the US’s action in Iraq, given its longstanding argument of Washington being an anti-Iraq actor. But even as the war’s outcome remains uncertain, Iraq appears to have witnessed an Iranian strategic victory.