For all the focus on the battle to capture Raqqa, the climactic battle against the Islamic State in Syria may be about to take place along a relatively unknown river valley miles away from the group’s self-styled “caliphate” capital.

Military commanders in Damascus, Tehran and Moscow are setting their sights on the Syrian city of Deir el-Zour and the surrounding Middle Euphrates River Valley as the battleground for the fight against the jihadi group, also known as ISIS or ISIL.

Located 90 miles from Raqqa, where U.S.-backed militias began their assault to retake the city last week, the fertile stretch of land along the banks of the Euphrates River is home to Madan, Deir el-Zour and other Islamic State redoubts. Many of the group’s leaders, including “caliph” Abu Bakr al-Baghdadi, have fled to the area.

Coalition commanders and Pentagon officials say the overall battle plan will address the Islamic State buildup in Deir el-Zour. But with all eyes fixed on Raqqa, it remains to be seen how Syrian-led operations, backed by Russia, will affect that long-term strategy.

Elements of the U.S.-backed Syrian Democratic Forces, or SDF, have borne the brunt of the fighting in and around Deir el-Zour. But the Raqqa operation is drawing focus away from the Middle Euphrates River Valley, leaving Syrian government forces and paramilitary groups backing the regime of President Bashar Assad to defend the area against Islamic State militants.

Damascus and minders in Moscow are having difficulty reining in Iranian Shiite paramilitaries tied to the regime. The Iranian troops are playing their own regional game in southern Syria, antagonizing U.S. forces who trained local fighters to carry out coalition operations against the Islamic State in the Middle Euphrates River Valley.

A long-standing deconfliction channel between Washington and Moscow, created to ensure the Russian-led mission to prop up the Assad regime in Syria did not interfere with U.S. efforts to defeat the Islamic State, failed to prevent a drone attack on U.S. and Syrian forces outside the coalition’s southern Syrian training base in Tanf.

A foreign drone reportedly bombed a joint U.S. and Syrian patrol outside the Tanf camp last week. The Pentagon confirmed there were no casualties from the attack but would not comment on whether the drone was operated by the Syrian armed forces or another foreign military.

Russian and Syrian leaders agreed not to enter the area surrounding the camp at Tanf, designating it as a “deconfliction zone.”

But Iranian-backed forces have attempted to breach those areas repeatedly over the past several weeks. The drone attacked Thursday as U.S. fighter jets launched airstrikes against Iranian forces trying to cross into the zone. It was the second U.S. airstrike against those forces in as many months.

Safe havens

Islamic State leaders, including al-Baghdadi, reportedly began fleeing Raqqa for Deir el-Zour, Madan and other areas in the Middle Euphrates River Valley en masse in May, officials at U.S. Central Command said at the time.

Their escape, likely prompted by the coalition’s advances in Mosul, convinced factions within the group’s leadership “from a military perspective, that this may not be tenable to hold on to Raqqa,” said a command official speaking on the condition of anonymity.

U.S. military planners, including top coalition commander Lt. Gen. Stephen Townsend, sought to expedite the Raqqa offensive in part to ensure top leaders of the Islamic State were not able to escape the coalition’s tightening noose around the city.

But outside factors, including ongoing tensions between the U.S. and Turkey over the Kurdish role in the operation, ultimately hindered efforts to speed up the Raqqa assault. A week into the operation to liberate Raqqa, questions over the true strategic importance of the city have only grown louder within defense and national security circles.

Islamic State-inspired attacks have rocked the United Kingdom, Singapore and elsewhere as the terrorist group loses its main pillars of power in Iraq and Syria.

Clashes between Islamic State fighters and Russian-backed Syrian forces are also heating up in the Euphrates Valley, giving Damascus and its backers in Moscow an opening to expand Mr. Assad’s grip on Syria.

Regime forces repelled over 1,000 fighters on Monday when the jihadis attempted to overrun the city’s airport and Syrian positions near Al-Tameen Brigade Base, local reports say.

The regime’s victory over anti-government forces in the rebel stronghold of Aleppo late last year has emboldened and increased Syrian and Russian presence on the Islamic State battlefield.

Since March, the regime has carried out offensives in Deir Hafer, an Islamic State enclave 30 miles east of Aleppo, and al-Bab. Government troops seized control of the main roadways leading from al-Bab into Raqqa in February.

That presence, seen as an attempt by the Assad regime to maintain sway over the country and gain leverage during ongoing peace talks, has confounded U.S. and coalition commanders trying to maintain order among the various forces battling the Islamic State in northern Syria.

Iran’s game

In southern Syria, Tehran is playing a similar but much more dangerous game with the United States in and around the Syrian-Iraqi border where U.S. forces are training local militias in the fight against the Islamic State.

Hundreds of Shiite paramilitary forces continue to amass around the Syrian coalition base at Tanf along the Iraqi border. The militias being trained by American special operations advisers, some of whom are former members of the Free Syrian Army, will spearhead coalition operations against the Islamic State in Der el-Zour and the Middle Euphrates River Valley.

While training continues apace, the massing of hundreds of Iranian militiamen and pro-regime forces around the base at Tanf have effectively cut off the main routes from the border areas into the valley, former U.S. Ambassador to Saudi Arabia Robert Ford wrote in a recent analysis for the Middle East Institute.

“This Syrian government dash effectively blocks the northeastern advance of the American-backed Syrian rebel forces leaving them bottled up in a Pusan-like corner,” wrote Mr. Ford, who is a senior fellow for the Washington-based think tank.

Clashes between U.S. forces and pro-regime forces tied to Iran may be the beginning of a smaller fight in the shadow of the Islamic State campaign.

Iranian-backed forces allied with the Assad regime are looking to join Shiite militias, known as Popular Mobilization Units, affiliated with Hezbollah and the Islamic Revolutionary Guard Corps fighting the Islamic State in Iraq, Mr. Ford said.

“Washington will face a dilemma if, as likely, the Syrian government chooses to escalate in eastern Syria in the weeks ahead,” he added.

With Mosul operations expected to wrap up within weeks, significant elements of the PMUs could end up flooding into Syria to take the fight to the Islamic State there, expanding Tehran’s influence in Syria, said David Pollock, a senior Middle East analyst at the Washington Institute for Near East Policy.

“They are already there in great numbers,” Mr. Pollock said last week at a terrorism symposium sponsored by the Foundation for the Defense of Democracies. He was referring to Iraqi PMUs joining forces with Shiite militias already in Syria in support.

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