The departure of US forces from Iraq will embolden Iran, experts say, warning that Tehran could seek to exploit a potential security vacuum to expand its influence in the smaller neighboring country.
Iran is already a key political and economic player in Iraq, where it backs powerful Shi'ite militias, some of which operate within the state security apparatus but have considerable autonomy.
The Islamic republic cast the end of the two-decade US military presence on September 30 as a major victory in its goal of driving American forces from the Middle East.
"Iran views any US weakness as a chance to push and be more aggressive, so I would expect to see the Iranians attempt to take advantage of the new status quo by flexing their muscles in Iraq," said Colin Clarke, executive director of the Soufan Center, a New York-based think tank.
Experts also warn the US pullout could provide an opening for the resurgence of the Islamic State (IS) extremist group, which has been significantly weakened but remains active in Iraq.
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Iran labeled the US exit from Iraq as a triumph for its so-called axis of resistance, Tehran's loose network of proxies and militant groups against archfoes Israel and the United States.
The Islamic Revolutionary Guards Corps, the elite branch of Iran's armed forces, said the pullout was a "massive achievement" for the axis and marked the end of "America's heavy and disgraceful defeat" in Iraq, in a statement issued on October 1.
US President Donald Trump hailed the withdrawal as a "victory" for Washington and Baghdad, adding it was "time for America to go home!"
The timing of the US pullout, which came amid the ongoing US war with Iran, has raised eyebrows, with experts saying it could send the wrong message to Tehran.
"Tehran will treat it as evidence that Washington is shedding a commitment under fire and as an opening to tighten its own grip in Iraq," said Farzin Nadimi, an Iran defense expert at the Washington Institute, adding Iran will see the pullout as "proof that pressure works."
When the United States and Israel launched an air campaign in Iran in late February, Tehran's proxy forces in Iraq joined the fray, carrying out attacks against US diplomatic and military facilities.
Iran itself has carried out waves of missile and drones strikes in Iraq's semi-autonomous Kurdish region in the north, where Iranian Kurdish opposition groups operate camps and offices.
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The so-called Islamic Resistance in Iraq, an umbrella organization of Shi'ite, Iran-backed armed groups, has been buoyed by the US withdrawal, which it portrayed as a victory that was forced by its attacks.
Some of the Iran-backed groups that form the Islamic Resistance in Iraq are also part of the Popular Mobilization Forces (PMF), an umbrella organization of mostly pro-Iranian militias that has been a part of the Iraqi Army since 2016.
Part of Iran's axis of resistance, some Iran-aligned groups have refused to disarm and hand their weapons to the state. Baghdad was forced to extend the original September 30 deadline for disarmament to June 2027.
Kataib Hizballah, one of the most powerful pro-Iranian militias in Iraq and a US-designated terrorist group, issued a threat to the Baghdad government on the eve of the US pullout.
"A government that doesn't work for the good of its people will be brought down by all possible means," a statement said.
Experts say groups like Kataib Hizballah will see the US withdrawal as a green light to expand their freedom of maneuver and presence in parts of the country where their militias operate.
That includes Iraqi Kurdistan, which borders northwestern Iran and was home to a US air base that closed on September 30.
"With US troops and air defenses gone, these groups have more room to pressure the Kurdish Regional Government, deepen their role in Iraqi security institutions, and use Iraqi territory for attacks on US partners," said Nadimi of the Washington Institute.
"The withdrawal of US air and missile defense assets from [Iraqi Kurdistan] is particularly seen as troubling. The United States can still strike from outside Iraq, but the on-the-ground constraint is gone," he added.
The Pentagon has said the United States will keep providing training and intelligence to Iraqi government forces.
Opening For IS Extremists?
Many Iraqis have welcomed the departure of US forces, although some are concerned over the readiness of government forces to protect the Middle Eastern country of some 48 million people.
The United States invaded Iraq in 2003 and toppled dictator Saddam Hussein, triggering years of war and sectarian strife from which the country is still recovering.
After withdrawing in 2011, the United States sent troops back to Iraq in 2014 to help fight IS extremists who had seized large swaths of the country and neighboring Syria.
Iraq's security forces have developed greater operational capabilities but lack air power and continue to reply on external support. Experts say the US pullout is likely to lead to more insecurity in Iraq, a situation that extremist groups are likely to try and capitalize on.
"IS will see this as an opportunity, and it remains uncertain whether and to what extent Iraqi security forces will be able to continue to keep the group on the ropes," said Clarke of the Soufan Center.
"It is hard to look at the present and not be reminded of the US withdrawal in 2011, which sowed the seeds for the rise of IS in the first place."