The departure on Wednesday, celebrated as a victory by Tehran and its allies, was agreed in 2024 under President Joe Biden and carried out by Donald Trump's administration even as it wages war against Iran, which wields influence in Iraq through proxies.
It comes 23 years after US-led forces toppled dictator Saddam Hussein, then triggered upheaval and chaos, culminating with the rise of Islamic State.
While the departure of US forces is broadly popular among Iraqis, some worry that security forces trained and funded by the United States still can't protect the country, where 4500 American forces died in more than 20 years of war.
"The lesson from the past two decades is that security vacuums can be extremely dangerous," said Jassim al-Bahadli, an Iraqi analyst.
US forces invaded Iraq in 2003 and went on to occupy the country, install a Shi'ite-led government and fight a years-long counterinsurgency - first by Saddam loyalists, then al-Qaeda - 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.
Around twice as many Americans died in "Operation Iraqi Freedom" as in the parallel war in Afghanistan, which ended with an abrupt US withdrawal in 2021.
In this year's conflict with Iran, seven of the 18 US service members listed as killed were in Iraq, the last an army sergeant who died during the controlled detonation of a drone at a base near Erbil in July.
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 triumph for its "axis of resistance" of allied armed groups across the region.
"What is further concerning is not necessarily direct Iranian intervention, but the possibility that Iran's proxies will feel increasingly empowered in the absence of a Western military presence. These groups may attempt to expand their role in shaping state policies and consolidating their presence within key institutions," said Baghdad-based political analyst Ahmed Younis.
Some Iraqi security officials, both in the federal government and the semi-autonomous northern Kurdish region, say the US pullout also risks giving new life to Islamic State fighters, who, despite the defeat of their self-proclaimed caliphate nearly a decade ago, retain a sleeper cell network.
At their height, the militants occupied a third of Iraq and slaughtered thousands of people, including hundreds they videotaped themselves gunning down inside a former US military base.
US troops had been providing Iraqi security forces with critical intelligence on Islamic State movements, aerial surveillance and reconnaissance.
"The most important question now is whether Iraq's security institutions have prepared a viable Plan B for the post-coalition phase," said retired Iraqi Army General Mohammed al-Jubouri, who led a brigade in the war against Islamic State.
"One of the worst-case scenarios would be Islamic State obtaining armed drones or improvised explosive drones at a time when Iraq still lacks sufficient air defence capabilities to counter such threats effectively."
