War in Iraq: Iran Allies Battle ISIS to Secure Border With Syria and Unite Forces

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Popular Mobilization Forces (PMF) ride on a tank during a battle with Islamic State group militants, at the Um Jaris village on the Iraqi border with Syria on May 29. The victory by the Iran-backed group has given Tehran a direct land route to Syria, but the U.S. has demonstrated its willingness to use military force to defend its own interests in Syria from both the local government and other rival powers. Stringer/Reuters

Iran-backed militias in Iraq have advanced against the Islamic State militant group (ISIS) on the nation's border with Syria, where the units hope to link up with a parallel anti-ISIS offensive run by the Syrian army and its allies.

Armed groups under the umbrella of the majority-Shiite Muslim Popular Mobilization Forces (PMF), also called Al-Hashd Al-Shaabi, have strategically pushed westward, battling the remains of ISIS' self-proclaimed Sunni caliphate along the Syrian border. The militias are part of an alliance that includes the Iraqi military, Kurdish forces and a U.S.-led coalition currently battling ISIS in its final Iraqi stronghold of Mosul. As partner forces engage the remnants of ISIS' control in Mosul, the PMF have successfully cut ISIS off outside the city and on Monday retook a number of villages on the Syrian border, Reuters reported.

Related: The Final Push for Mosul: ‘Matter of Time' until ISIS flag falls in Iraq's second city

"This will be the first step to the liberation of the entire border," Ahmad al-Asadi, a spokesperson for the PMF said, according to the Associated Press. "This victory will also be an important incentive for the Syrian Arab Army to secure the entire border from the Syrian side," he added.

RTX3858X Popular Mobilization Forces (PMF) ride on a tank during a battle with Islamic State group militants, at the Um Jaris village on the Iraqi border with Syria, Iraq May 29, 2017. The victory by the Iran-backed group has given Tehran a direct land route to Syria, but the U.S. has demonstrated its willingness to use military force to defend its own interests in Syria from both the local government and other rival powers. Stringer/Reuters

Iranian Major General Qasem Soleimani, the iconic head of Iran's elite Islamic Revolutionary Guards Corps, was reportedly pictured with PMF forces at a border site in northwestern Iraq shortly after militias had secured it, signaling the importance of the event. Iran has also sent advisers and supported powerful paramilitary movements such as the Lebanon-based Hezbollah that back the Syrian military as it attempts to restore control of its side of the Syria-Iraq border. Iran is not only an ally of the U.S.-backed Iraqi government, but also of the Russia-backed Syrian government of President Bashar al-Assad, who faces a six-year insurgency from ISIS and various other rebel groups, some of which are backed by the U.S.

On the other side of the Iraq-Syria border site recently taken by the PMF lies territory held by the Kurdish-majority Syrian Democratic Forces (SDF), made up of Arabs and ethnic minorities. While the SDF has become the U.S.' main ally against ISIS in Syria, the group remains relatively neutral toward the conflict between Assad and Syrian rebels and has at times assisted Syrian army operations.

At another border crossing hundreds of miles away in the easternmost stretch of Iraq, however, the PMF have also made advances that could lead to potential conflict with militants supported by the U.S. Fighters for the PMF group known as Kataib Al-Imam Ali were seen on social media preparing for what they said was an incursion into Syria toward the border crossing of al-Tanf situated in the Badia region where Syria, Iraq and Jordan all meet. Much of the region is currently held by Syrian opposition groups backed by U.S. Special Forces and operating under the banner of the Free Syrian Army, a loose coalition of West-backed rebel groups.

The U.S. launched an airstrike against the Syrian army and allies earlier this month after claiming its vehicles came within 18 miles of the U.S. operations base near al-Tanf, sparking outrage from the Syrian government. The Syrian government has referred to the U.S.-backed SDF's struggle against ISIS as legitimate, but rejects any U.S. presence not coordinated with the local government or its Russian ally.

The U.S.' continued presence in southern Syria, as well as its support for anti-Assad groups, has frustrated the Syrian military and its allies' attempts to recapture the vast eastern swathes of the country not under government control. While both the Syrian government and the U.S. aim to defeat ISIS, Washington has challenged Assad's legitimacy over allegations of chemical weapons use against civilians and his ties to Iran, a traditional foe of U.S. policy in the Middle East.

The U.S. has, however, reluctantly accepted the PMF's role in defeating ISIS in Iraq, despite the militia's ties to Iran and even the direct presence of Iranian military officials on the ground to advise and assist. While still comprised of mostly Shiite Muslim militias, the PMF receives direct backing from the Iraqi government and includes some Iraqi minority groups, such as Sunni Muslims, Christians and Yazidis.