On May 29, the Hashd al-Shaabi, or Popular Mobilization Units (PMU), a group of Shia militias that are part of the Iraqi government’s security forces, reached the Iraq-Syria border. Straddling this strategic corridor in northern Iraq, which stretches east to Mosul and then south to Baghdad, allows the militias to dictate Iraq’s future war aims against ISIS to the south, as well as its policy in regards to the Kurds north of the new front line. For U.S. policymakers who are leading the coalition against the Islamic State, the role of the PMU and the Iranian influence it projects are a key concern for the future of Iraq, Syria and the region. The Shia militias’ next moves have the potential to affect the United States’ partnership with Syrian rebels near Jordan, and with U.S. Kurdish allies in northeastern Syria.

The PMU have played a key role in the war since June 2014, when Ayatollah Ali al-Sistani issued a fatwa and call to arms for Iraqi Shia to fight ISIS. The militias also control swaths of territory liberated from ISIS. In early April, when I last visited Mosul, the roads leading to the city’s front line against ISIS were festooned with checkpoints run by various PMU affiliates. Shia religious flags adorn the PMU vehicles, and posters sometimes depict Iran’s Supreme Leader Ali Khamenei.

The Iraqi government has attempted to keep the militias out of the direct battle for Mosul, preferring a partnership with the U.S.-led coalition, which does not officially work with the PMU. However the militias were permitted freedom of action to the west of Mosul. They surrounded Tal Afar, a strategic town occupied by ISIS, and cut off ISIS supply lines from Syria. In mid-May they launched an offensive aimed at Baaj, a town where some reports have claimed ISIS leader Abu Bakr al-Baghdadi may be hiding. The PMU bypassed the town to the north, between ISIS and the Kurdish peshmerga in Sinjar, and reached the Syrian border on May 29.

The Badr Organization’s commander, Hadi al-Amiri, announced the arrival of his militia force at the border to Iraq’s Alsumaria television. Al-Amiri, like many PMU senior officers, served alongside the Iranians in the Iran-Iraq War in the 1980s. Photos posted online showed Shia militiamen celebrating next to signs pointing towards Homs in Syria. Iran’s PressTV showcased the success, noting that the Shia militias would cut ISIS’s “vital supply line” to Syria. Qassem Soleimani, the leader of Iran’s IRGC Quds Force, was seen in photos near the Syrian border.

The new corridor that the PMU has carved out links Syria—via Mosul and Baghdad—with Tehran. It has redrawn the map of Iraq, placing the Shia militias solidly astride the country. While the Iraqi Army and its Interior Ministry’s Federal Police have done the heavy lifting in the battle for Mosul, the militias stand to gain the most. Iraqi prime minister Haider al-Abadi flew to Mosul on May 29 and posted on Twitter that he came to “oversee liberation operations and meets with ISF [Iraqi Security Forces], PMU commanders.” Abu Mahdi al-Muhandis, a senior PMU leader, was in the photos Abadi tweeted. Symbolically, this shows that the PMU is the power behind the throne, and that behind that power lies Iranian influence.

This stokes fears in the Kurdistan Regional Government (KRG) capital of Erbil, where Kurdish officials have been planning a referendum but are not confronted with a powerful Shia force that has opposed plans for independence. According to Rudaw, a Kurdish media outlet, the General Command of the Peshmerga issued a statement warning the PMU that any who “transgress against the land of Kurdistan” would be opposed—that would “beat their heads against the mountains of Kurdistan.”

The Kurds, who liberated Sinjar in November 2015, have been divided near the Syrian border due to infighting between PKK-affiliated Kurdish groups and Erbil’s peshmerga. The PMU offensive caught them off guard. Many of the villages the PMU liberated were previously home to the Yazidi minority and the center of ISIS atrocities in 2014. Kurds see them as part of the KRG and the areas that it disputes with Baghdad—disputes that predate the arrival of ISIS. With the Shia militias running them, Erbil’s plans for any further operations in Sinjar are stymied. The PMU also finds itself with new neighbors: a Yazidi force connected to the PKK that controls a part of the Syrian border, and the Syrian Democratic Forces on the other side of the border. The SDF are close allies of the United States, and the Pentagon has been sending equipment, arms and soldiers to fight alongside them for the liberation of Raqqa. That means that U.S. allies who are suspicious of the PMU now share a border with it.

The PMU can’t move into Syria without clashing with the SDF, and it can’t move north without fighting the Kurds. Its immediate goal is to dash along the Iraqi border to the border town of Qaim in Anbar Province. This is important because ISIS’s recent car bomb in Baghdad, during Ramadan’s early days, allegedly came from Anbar. As Mosul falls, the PMU wants to cut off another jihadist tentacle. But as the PMU reaches Qaim, it will come closer to areas controlled by the Syrian regime of Bashar al-Assad and by Syrian rebel groups supported by the United States.

This is a combustible mix, because the United States recently launched airstrikes on May 18 against Syrian regime forces that were heading for al-Tanf, near the border with Jordan and Iraq. With the PMU moving south and the Syrian regime moving east, the two forces, both allied with Iran, will want to link up. This would create what commentators have deemed a “Shia crescent” or Iran’s “road to the sea.” Fabrice Balanche at the Washington Institute for Near East Policy has drawn attention to Iran’s quest for this corridor in recent articles. Jonathan Spyer, director of the Rubin Center for Research in International Affairs at the Interdisciplinary Center Herzliya, noted in a recent piece in the Jerusalem Post that this raises the prospect of Lebanese Hezbollah having a land route to the PMU via Syria.

For the United States, the challenge here is that Iran is quickly filling the void left by ISIS in Iraq. Although the SDF and Kurdish region of northern Iraq may be a counterbalance to that, the role of the PMU in Iraq is of concern. The idea that they could become like Hezbollah or the IRGC is now a very real risk. With a hundred thousand men, and dozens of militias, they will end up controlling Sunni areas of Iraq that were the heartland of the insurgency against U.S. forces after 2003. Allegations of their human-rights abuses do not bode well for the future of those areas. Strategically, they appear more and more to operate as a corridor for Iran’s larger plans in the region. That may be a corridor of influence, or it can be a physical road to transfer arms, both to serve Assad’s interests and to undermine U.S. policy in Syria and Iraq.

The PMU’s recent offensive has redrawn the map of Iraq; its next move will have serious consequences for the region.

Seth J. Frantzman is a Jerusalem-based journalist and research associate at the Rubin Center for Research in International Affairs in Herzliya​.