Members of the Abbas combat squad, a Shiite militia group, carry a picture of Grand Ayatollah Ali al-Sistani during a parade in Basra, Iraq, on September 26, 2015.Nabil al-Jurani / AP

In June 2014, Grand Ayatollah Ali al-Sistani, one of the leading Shiite clergyman in the world, called on all able-bodied Iraqis to defend their country against the Islamic State. Iraq’s U.S.-trained armed forces had collapsed, fleeing the advance of ISIS as it seized Mosul and much of northern Iraq. Sistani’s fatwa mobilized a 100,000-strong fighting force known as the Hashd al-Shaabi, or Popular Mobilization Forces (PMF), whose mostly Shiite fighters were instrumental in the fight against ISIS. The PMF is comprised of multiple Shiite militias who were established after 2014 as volunteer groups that took up arms in response to Sistani’s fatwa, filling the void left by the collapse of the Iraqi army. The majority of these groups are aligned with the Iraqi state and take their orders from the Iraqi government.

But residing within the PMF are Iran-aligned groups who have become the Forces’ most-powerful militias. While technically they have been under Baghdad’s command since 2016, in reality, they answer to their sponsors in Tehran. These groups have long exploited conflict and disorder in Iraq since the toppling of the Baath regime, while also expanding Iran’s influence in the country. They have been accused of sectarian atrocities that helped lay the groundwork for groups like ISIS and played a critical role in the bloody 2006 war between Arab Sunnis and Shiites. They have violently resisted attempts by the Iraqi state and the United States to disarm them. Since the emergence of ISIS and Sistani’s fatwa, these groups have exploited the security vacuum and the weakening of Iraq’s conventional forces to further consolidate their hold. Now, they seem poised to translate their wartime popularity into political gains in the coming elections in May, when they will contest the elections as the al-Fateh (or “Conquest”) bloc.

With ISIS vanquished and Iraq’s security forces reconstituted and reorganized (thanks to U.S. training and support), some expected that Sistani would revoke his fatwa and dismantle the PMF last December. But Sistani cannot simply dissolve the PMF, a state institution that provides livelihoods and prestige to its fighters. Doing so would spark a public backlash and undermine Iraq’s Shiite clerical establishment. Neither can he confront Iran’s proxies alone as they, and Shiite militias more generally, have proved themselves formidable actors. (The United States tried with more than 100,000 troops during the occupation and failed.)

But left untouched, Iran’s proxies will continue exacerbating sectarian tensions that could very well enable the resurgence of ISIS. Sistani will have to confront them eventually—but not by himself, and not in the way some may expect.

Essential to the Shiite militias’ survival over the years has been their capacity to adapt to the political and legal constraints imposed on them. They either attach themselves to longstanding parties or rebrand themselves as socio-cultural movements that provide social services to local, often destitute communities. Groups like Asaib ahl al-Haq, which has been complicit in attacks on U.S. and Iraqi personnel and civilians, were established by Iran after 2003 and have since transitioned into powerful armed groups that enjoy access to state institutions and resources, yet continue to function autonomously. Iran-aligned Shiite militias have, in essence, established themselves as Iraq’s version of Lebanon’s Hezbollah: socio-cultural movements with a military and social-welfare wing that operates independently of the state.

Asaib al-Haq is not the only Shiite militia in Iraq that has benefitted from Baghdad’s resources without ever submitting to its control or to civilian oversight. Another prominent example is the Badr Brigade, an organization established by Iran during the 1980s Iran-Iraq war. While it began as a militia, since 2003 it has controlled Iraq’s interior ministry, and today commands a 37,000-strong federal police force. The Iraqi interior ministry answers not to the prime minister, but to the Badr hierarchy, led by its leader Hadi al-Ameri, who fought alongside Iranian forces during the Iran-Iraq war. He also now leads the PMF. While the Badr Brigade is the only Iranian proxy that controls an Iraqi ministry, this may not be the case for long.

Iraq’s army is not strong enough to confront these groups. But Sistani has the credibility to lead such an effort. He has long served as a critical check on the power of Iraq’s corrupt ruling elite. After the toppling of Saddam, he ensured that the process of writing a new constitution would be led by an elected assembly, rather than by Washington’s favored Iraqi elites. While he failed to prevent sectarian war, his call for calm, restraint, and unity, helped ensure that the conflict did not transform into a genocide against Sunnis. In August 2014, only two months after his call to arms against ISIS, he forced out then-prime minister Nouri al-Maliki, whose corrupt and authoritarian rule led to the collapse of the Iraqi army.

The Ayatollah has historically resisted Iran’s efforts to export its theocracy to Iraq. He has criticized and will continue to pressure Iran’s proxies through his sermons. While Sistani’s record suggests he’s up for combatting Iran’s proxies, he’ll need help. That may come from nationalist, anti-Iran voices like Muqtada al-Sadr and his followers. Both al-Sadr and Sistani have backed Iraq’s anti-corruption protests, which have been led by members of civil society, including human rights organizations and other NGOs. Hundreds of thousands of Sadr’s supporters, for instance, mobilized in 2016 to call for reform and an end to sectarian governance. Many chanted anti-Iranian slogans. Sadr’s visits to the Gulf have also strengthened Iraq’s ties with the Arab Sunni world. These relationships could establish cross-sectarian alliances to contain Iran-aligned factions.

The United States has an important role to play in all this. Maintaining its military presence in Iraq will help contain Iran’s proxies, so long as it does not weaken Tehran’s rivals like the Kurds and Sunnis. This is exactly what happened last October, when al-Abadi’s forces and Iranian proxies reclaimed oil-rich Kirkuk and its surrounding areas from the Kurds. Indeed, al-Abadi, who has been billed in some quarters as America’s man in Baghdad, has relied on Iran-backed militias to maintain Baghdad’s control over territories that are disputed with the Kurds.

As a result of that onslaught, Iran’s proxies now control Kirkuk and other strategically vital towns and cities. With every inch of territory Iran’s proxies acquire, their influence becomes stronger in the rest of Iraq. Al-Abadi has even contemplated an alliance with Iran’s proxies, a move that has been criticized by Sistani and the Najaf religious establishment, al-Sadr, and Arab Sunni and Kurdish factions. Washington would do well to heed the lessons of the past and avoid creating a strongman in Baghdad who may one day turn his back on the United States.

To contain Tehran, the United States could also help prevent these groups from appropriating the $1 billion allocated to the PMF from the Iraqi national budget, and curb their access to the billions of dollars that the international community intends to contribute to the reconstruction of Iraq. These resources enhance the battlefield superiority of Iran’s proxies. They also allow them to shape Iraq’s political system according to their own ideologies, while molding the fabric of its society through its sophisticated propaganda.

Sistani, who champions a pluralistic, representative Iraqi state, can go a long way toward containing Iran’s proxies. On his own, however, he can only do so much.