Recent developments in the eastern half of Syria suggest we are entering a new phase in the country’s conflict. As the regime of President Bashar al-Assad consolidates itself militarily in western Syria, and as the Islamic State faces an imminent offensive in Raqqa, attention is now shifting to the long and contentious border area with Iraq.

That’s not surprising. Syria’s east always had vital importance for the Assad regimes, abutting two of the country’s main regional rivals, Iraq and Turkey. It is also home to a large Kurdish community that has long had an uneasy relationship with Damascus. In a recent Carnegie paper on developments in Hasakeh governorate in the northeast, Kheder Khaddour underlined that for the Syrian regime a security imperative traditionally prevailed in the region, to the extent that Syrian intelligence officers were often promoted according to how they managed the situation there.

In anticipation of the Islamic State’s defeat in and around Raqqa, many outside actors appear to be preparing to take advantage of the aftermath. If the Islamic State loses the city, attention will shift to the group’s last strongholds further to the southeast, in the area of Deir Ezzor, Mayadin, and Albukamal. Whoever can defeat the group in this region would effectively take control of a large segment of the Iraqi-Syrian border.

The Assad regime and its Iranian and Russian allies regard such control as a strategic priority. Deir Ezzor governorate is rich in oil, a vital factor for a cash-strapped Syrian government. But taking back the border area would also represent a highly symbolic act for Assad, showing that he is slowly reconstituting the pre-2011 Syria that he inherited from his father.

For Iran, whoever dominates the frontier with Iraq will determine how easily the Islamic Republic can resupply and reinforce its allies in Syria and Lebanon, particularly at a moment when it seeks to build up its deterrence capability around Israel. The border is an essential hinge in Iran’s efforts to maintain its influence throughout the Levant.

It’s with this in mind recently that a Syrian regime military column that included Hezbollah and Iraqi Shi‘a combatants sought to move on Tanf, the border crossing between Syria and Iraq that lies near the Jordanian border. Tanf is where U.S. special forces are based to train Syrian rebels. When the column entered a 25-kilometer radius the United States had defined as an exclusion zone to protect its forces, it was bombed by U.S. aircraft.

This incident followed a rise in tensions around Tanf. Syria and its Iraqi and Lebanese allies have deployed surface to air missiles there to use, if needed, against coalition aircraft. On May 29, a U.S. Defense Department spokesman confirmed that Iranian-backed militiamen were massing near Tanf, adding that U.S. forces would defend themselves.

This corresponded with reports on May 30 that Syrian rebel groups in southern Syria had received weapons supplies from the Pentagon and the Central Intelligence Agency. Syrian rebel sources said the weapons had been sent to help repel efforts by the Syrian regime and its allies to open the road between Iraq and Syria, which passes through Tanf. Yet Tanf can also play another role, as it represents a point of departure from where forces can move to recapture Islamic State-controlled areas in Deir Ezzor governorate.

From the U.S. reaction to events around Tanf, it appears that Washington wants to be in on any eventual offensive against the Islamic State in Deir Ezzor, but also that it would like to keep an eye on Iran’s actions in southeastern Syria. Both objectives are related. The U.S. is training young men from Deir Ezzor in Tanf to use in future operations against the Islamic State. This suggests that it seeks to expand its sway along the Syrian-Iraqi border, which is unacceptable to Iran. No wonder. The standoff in southeastern Syria only really makes sense if we assume that Washington also intends to hinder Iranian moves and gain leverage that potentially allows it to shape a political endgame in the Syrian conflict.

That is why Russia seems so uneasy with the situation in Tanf. On May 31 Russian aircraft bombed U.S.-backed rebels advancing on the positions of pro-Iran militias at the Zaza checkpoint, northwest of Tanf on the Damascus-Baghdad highway, amid reports that a military column flying Russian flags had arrived in the city of Der‘a. It could also explain why Moscow appears to be near agreement with Iran to station Russian aircraft at an Iranian airbase in Hamadan province. If this were to happen, it would not only allow Russian aircraft to operate in eastern Syria, it would also send a strong message that Russia and Iran (as well as Iraq) are in agreement in their refusal to allow outside, particularly U.S., involvement in defining a political solution in Syria.

Until now, the Trump administration’s statements about wanting to diminish Iran’s role in Syria have been general. Events in the southeast are adding substance to that commitment. However, given Iran’s multiple alliances, the odds are against the U.S. Perhaps the greatest paradox, one nobody in Washington will mention, is that in the greater game between Iran and the U.S., the Americans do not want the Islamic State in Deir Ezzor to be defeated by anyone but themselves—certainly not by Tehran’s allies.