Iraqi security experts say the withdrawal, to be finalised on Wednesday, could give Tehran and its powerful allies free rein, while also allowing other US enemies to stage a comeback, including Islamic State.
"Iran stands to benefit strategically from the coalition's departure because it removes a military presence that for years acted as a counterweight to Tehran's influence in Iraq," said Jasim al-Bahadli, an Iraqi security analyst specialising in studying armed Shi'ite groups aligned with Iran.
He predicted that the pro-Iranian militia groups would not only portray the departure as a victory for their movement, but also "seek to translate that momentum into greater influence over security and political decision-making".
Abu Mojtaba al-Yasiri, a commander in the Islamic Resistance in Iraq, an umbrella group of Iran-backed armed factions, described the US withdrawal as "a historic victory for the Iraqi Islamic resistance groups and honourable Iraqis, and a crushing defeat for the American project".
"On September 30, we will celebrate the turning of a black page in Iraq's history and the failure of the occupation to impose its will on our country," he told Reuters.
"Iraq has proven that its sovereignty cannot be secured by foreign forces, but by the resistance and the will of its people."
While the eventual departure of US forces is broadly popular among Iraqis, those who worry about the strength of the Iran-backed groups fear the withdrawal is premature.
"We believe Iraq is still not fully ready to take over the security file at this time," said Abdulrahman al-Zobaie, a Sunni tribal leader in Fallujah, once a bastion of anti-American resistance.
"All Iraqis aspire to full sovereignty, and no Iraqi would dispute that. But I believe that the (US-led coalition) leaving in this way and creating a vacuum is what all Iraqis fear."
American forces invaded Iraq in 2003 to topple Sunni Muslim dictator Saddam Hussein, and went on to occupy the country, install a Shi'ite-led government and fight a years-long counterinsurgency before withdrawing at the end of 2011.
They returned less than three years later to help Iraqi forces defeat the Sunni militant group Islamic State in a major war from 2014 to 2017.
Iran, which says its ultimate aim is to force American forces to leave the entire Middle East, casts the US withdrawal from Iraq as a victory for its "axis of resistance" of allied armed groups across the region.
"Lasting security will prevail in Iraq" once the Americans leave the country and the region, the deputy chairman of the National Security Commission of Iran's Parliament, Amir Hayat Moqaddam, said on Monday.
