After a months-long siege by the Islamic State group, Iraq is about to retake the Sunni city of Tikrit, thanks to an army largely made up of Shiite militias backed by Iran. Tehran’s influence in Iraq has been rapidly increasing through a widespread network of proxy Shiite militias whose presence could tear open old wounds for the country’s Sunni population.

The offensive began on Monday and is one of the largest and most successful pushes against the militant group also known as ISIS. When ISIS declared a "caliphate" in Iraq this summer, the Iraqi government requested that the U.S. support the Iraqi Armed Forces on the ground and lead an air campaign to eradicate the militant group. Despite this partnership, the Iraqi army passed up on U.S. support in Tikrit, in favor of the growing network of Shiite militias.

“We are fully aware of the operation, but the Iraqis did not request our support for it,” Pentagon spokesman Colonel Steve Warren said. “Our presence in Iraq is at the request of the Iraqi government. We are there to advise them, to assist them, to support them, when they ask for it.”

The U.S. has ramped up the number of military trainers in Iraq as ISIS has continued to advance toward the capital of Baghdad, but Iraq’s military forces are still not enough to take on ISIS and are severely outnumbered by the Iran-backed militias that can now muster between 100,000 to 120,000, the Washington Post reported.

“If you look at the situation on the ground [in Tikrit], a huge chunk of the forces going in are Iraqi Shiite militia and these are the ones who are controlled by the Iranians,” said Phillip Smyth, a University of Maryland researcher who studies Lebanon and Shiite militias. “It would seem as if there’s far more control on the Iranian side since they’re using their organizations and their groups. These groups are generally not subservient to the Iraqi state.”

Roughly 30,000 fighters were part of the offensive that began on Monday and only half were Iraqi military, according to the Washington Post. About 2,000 combatants were Sunni tribal fighters and the remaining 13,000 were likely Shiite militia forces.

Iranian Gen. Ghasem Soleimani was reportedly in Iraq on Tuesday overseeing the offensive in Tikrit. Soleimani commands the Iranian Revolutionary Guard's Quds Force, one of the most powerful branches of the Iranian military. Iranian opposition groups estimated last year that there were roughly 7,000 Quds forces on the ground in Iraq both fighting and training new militia forces.

In 2012, Soleimani reportedly requested that 5,000 Iraqi Shiite volunteer fighters be deployed to Syria at the start of the civil war to support President Bashar Assad’s army. Many of those fighters came back to Iraq to join the fight against ISIS. Others ran for government positions under former Shiite Prime Minister Nouri al-Maliki. Maliki’s decision to step down last year was a blow to Iran’s influence in the country, but the Shiite regime has since been able to grow its proxy groups.

“A trend in the development of Iranian proxies is the creation of seemingly new groups characterized by unified ideology and loyal, proven personnel,” according to the “The Shiite Jihad in Syria and Its Regional Effects,” written by Smyth and published in the Washington Institute for Near East Policy.

There are at least 37 Iraqi Shiite militias with some degree of relationship to Iran, and 13 groups follow this trend. The militias are mostly made up of Shiite Iraqi combatants who have been trained in Syria, Lebanon and even Iran. At least six of Iraq’s Shiite militias have ties to Lebanon’s Shiite Iran-backed organization Hezbollah and at least nine have ties to the Badr Organization, an Iraqi political party whose military wing is commanded by Iran.

Among the Shiite militias who participated in Monday’s offensive is Kataib Hezbollah, a U.S.-designated terrorist group with ideological ties to Hezbollah in Lebanon. The group announced its participation in the operation through its website and posted videos and photos from the front lines in Tikrit to its social media pages.

“Tomorrow we will launch a system of advanced missiles that are specially made ​​for the battle to liberate all Tikrit and Salahuddin province,” Kataib Hezbollah spokesperson Jaafar al-Husseini Aksari said, according to the group’s Facebook page.

Kataib Hezbollah, along with Iran-backed Asaib Ahl al-Haq, were a driving force behind the August 2014 Iraqi offensive to break the ISIS siege in Amerli, according to Reuters. Shiite militias fought alongside Kurdish peshmerga forces, who now receive arms and funding from the U.S.

Iraqi militia group Kataib al-Imam Ali also reportedly participated in the offensive to retake Tikrit, but was responsible for the road toward Tikrit in in Salahuddin, according to a statement posted to the group’s website. The variety of Shiite brigades involved in this operation underscores the increasing Shiite presence in a largely Sunni area.

“Shiite militias have been taking advantage of the atmosphere of lawlessness and impunity to abduct and kill Sunni men, seemingly in reprisal or revenge for IS attacks and at times also to extort money from the families of those they have abducted,” according to an Amnesty International report on Shiite militias in Iraq.

Tikrit is a predominantly Sunni city -- and the hometown of Saddam Hussein -- where residents have long been victims of sectarian crackdowns from Iraq’s Shiite leaders and militias. During his time as prime minister, Al-Maliki was widely blamed for violent crackdowns against Iraq’s Sunni population. He stepped down last year.

“Just because there are a few token Sunni groups who are coming along that is not going to change that this will be viewed as a fairly sectarian push,” Smyth said. “You have to think about how the residents there are going to think of this. ISIS has a good handle on it, but this is going to be viewed as very sectarian.”