After the 2003 invasion of Iraq by the United States and its allies, The Coalition Provisional Authority disbanded the Iraqi military, security, and intelligence infrastructure of President Saddam Hussein and began a process of "de-Ba'athification".

[26][27] This move became an object of controversy, cited by some critics as the biggest American mistake made in the immediate aftermath of the fall of Saddam Hussein[28] and as one of the main causes of the rise of the Islamic State.

[29] The Badr organisation, which fought alongside Iran in the Iran-Iraq war, was seen as a U.S. asset in the fight against Ba'athist partisans because of their opposition to Saddam Hussein.

Shortly after the fall of Baghdad, Badr forces and other militias with close ties to Tehran reportedly joined the newly reconstituted army, police, and Interior Ministry in significant numbers.

Iranian Brigadier-General Massoud Jazayeri stated that Iran could best help Iraq by providing it with direction on its "successful experiments in popular all-around defence" that included "mobilising masses of all ethnic groups.

[2] Press agency Reuters believes that since mid-June, Iran has had members of its Islamic Revolutionary Guard Corps (IRGC) fighting on the ground in Iraq against ISIL, though it does not give an estimate of their numbers.

[51] By the end of June, according to American officials, Iran had established a control center at Al-Rasheed Air Base in Baghdad and was flying a small fleet of Ababil drones over Iraq, and an Iranian signals intelligence unit had been deployed at the airfield to intercept electronic communications between ISIL fighters and commanders.

[54] On 21–22 August, according to Kurdish sources, hundreds of Iranian soldiers, allegedly from Iran's 81st Armored Division,[55] helped Peshmerga to take back Jalawla in Diyala Governorate from ISIL.

[6][58] By early September, according to Business Insider, Iranian Quds Force personnel were deployed to Samarra, Baghdad, Karbala, and the former U.S. military post known as Camp Speicher.

In early October, the Islamic Republic of Iran News Network (IRINN) published a picture of Soleimani, purportedly on some battlefield alongside Kurdish Peshmerga.

[54] The Operation Ashura included Shi'ite paramilitary groups such as Asa'ib Ahl al-Haq, Kata'ib Hezbollah and the Badr brigades along with their leader Hadi Al-Amiri.

[66] US rear Admiral John Kirby affirmed on 3 December that he had "indications that [Iran] did indeed fly air strikes with F-4 Phantoms" targeting ISIL positions in the eastern Iraqi province of Diyala.

[72] American news website The Long War Journal states without revealing its sources that Iran has sent Islamic Revolutionary Guard Corps (IRGC) and Basij members to fight ISIL in both Syria and Iraq.

Hadi al-Amiri, leader of the Badr Organization, said, "If it were not for the cooperation of the Islamic Republic of Iran and General Suleimani, we would not today have a government headed by Haider al-Abadi in Baghdad".

[91] During the Second Battle of Tikrit, Hadi al-Amiri said US has failed to live up to its promises to help Iraq fight ISIL, unlike the "unconditional" assistance being given by Iran.

"[94] In addition to the aforementioned involvement, Iran has been even more engaged in the fight against ISIL, by providing financial and military support to the predominantly Shiite militias of the Popular Mobilization Units.

"[96]In July, Secretary of Defence Chuck Hagel offered further explanation, "We are aware of the Iranian and Russian efforts to help the Iraqis, but we are not involved in coordinating any missions.

[100] At the end of March 2015, the chairman of the joint chiefs of staff of the US Armed Forces, Gen. Martin Dempsey, stated that Iran's overt role in the Second Battle of Tikrit (March–April 2015) could be positive and could only become problematic if it descended into sectarianism.

Nonetheless, Khamenei also noted that some Iranian government officials were "not against" cooperation with the United States, explaining that he had personally quashed the suggestion of joint action against ISIL.

[112] Despite the cool reception given by Iran and its allies to the United States intervention, some observers believe the U.S. coalition will eventually be forced into brokering an alliance with Tehran.

Eyal Zisser of Tel Aviv University has explained that "the West is helpless and does not know what to do against the Islamic State" and that it will ultimately conclude it has no choice but to ally with Iran.