As ISIS retreats in Syria, the U.S. and Iran scramble for control

BEIRUT - U.S. and Iran-backed forces are locked in a race to take Islamic State strongholds in southeastern Syria and seize a stretch of land that will either cement Tehran's regional ambitions, or stifle them.

The scramble for pole position in Deir al-Zour province is likely to be one of the most consequential fights against the extremist group in Syria, posing a regional test for President Donald Trump as his administration turns up the rhetoric against Iran.

While the battle for the Islamic State's most famous Syrian stronghold of Raqqa is heating up, there are signs that an offensive to seize Deir al-Zour will be tougher, and have greater consequences for the group's long-term survival as a force holding significant territory.

On the Euphrates River between Raqqa and the Iraqi border, the city of Deir al-Zour is the largest urban center in eastern Syria. Victory for Syrian and Iran-backed forces there would give Tehran control of a large swath of the Syrian-Iraqi border, securing a land route through Iraq and southeast Syria to Damascus in the southwest, and on to its proxy, Hezbollah, in Lebanon.

For the United States, gaining control of Deir al-Zour would give it a bargaining chip for the future and demonstrate to regional allies its willingness to challenge Iran, after Trump promised to roll back the country's 'rising ambition.”

'The weakening of ISIS was always going to open a race for territory, dominance and influence. The aggressive tone coming from Washington incentivizes Iran to speed up its operations,” said Emile Hokayem, a Middle East analyst at the International Institute for Strategic Studies. 'The problem is that even what the U.S. sees as limited goals clash with more ambitious Iranian ones.”

On Tuesday, the United States clashed directly with Iranian proxies for the second time in a month, bombing pro-government militiamen as they advanced on an outpost used by U.S. Special Forces at al-Tanf, a key border crossing near Syria's southeastern border with Iraq.

The airstrikes have raised fears of retaliation in battle zones such as Iraq, where U.S. and Iran-backed forces are both fighting the Islamic State.

Experts say the Islamic State has moved senior leaders into Deir al-Zour, along with a growing number of foot soldiers, as internationally backed forces move in on Raqqa and Mosul, the group's so-called capitals in Syria and Iraq.

To reach the province, both sides are moving through Syria's vast southern desert as they head for the Islamic State-held town of al-Bukamel.

The race for position began last month after Russia, Turkey and Iran agreed to a cease-fire agreement that covered four parts of the country. Rebel commanders and Western diplomats believe the deal was intended to help Syrian President Bashar Assad and his allies concentrate resources in the east as they struggled to hold ground on multiple fronts.

'We see the link clearly now. Accepting those de-escalation zones meant the regime and its allies were able to relax and move resources,” said Abu Waleed, a commander with the U.S.-backed rebel group Usoud al-Sharqiya.

The retaking of the oil-rich Deir al-Zour region could give the Syrian government a financial boost and help diminish Assad's economic dependence on Iran and Russia, which have bankrolled his fight against an armed rebellion that began in 2011.

The Islamic State's capture of much of eastern Syria deprived the government of oil revenue. State production has dwindled throughout the war, from some 387,000 barrels per day to less than 10,000 now.

This weekend, the race accelerated as rebel commanders said that U.S. Special Forces had helped them build a new forward operating base about 40 miles northeast of al-Tanf. In photographs circulating on social media, a U.S. flag fluttered in the wind as fighters manned truck-mounted weapons.

The U.S. military said last week that it had bolstered its 'combat power” in southern Syria, warning that it viewed Iranian-backed fighters in the area as a threat to nearby coalition troops fighting the Islamic State.

U.S. Army Col. Ryan Dillon, a spokesman for the U.S.-led coalition in Iraq and Syria, told reporters Thursday that Iranian-backed forces were 20 miles within a 'deconfliction” zone they had declared a week earlier in an attempt to de-escalate tensions and holding firm near the U.S. base at al-Tanf.

U.S.-led coalition aircraft, he said, dropped leaflets asking the militiamen to leave. Dillon added that the U.S. military has communicated its concern to the Syrian forces through the Russian deconfliction line but had not yet given them an ultimatum.

The U.S.-backed force stationed in the southern desert still is limited, experts say. 'While the U.S. can rule the skies, it has little allied manpower at this point. It could recruit more and expand its footprint, but it would do so in inhospitable places where its rivals have an edge,” Hokayem said.

As Iraqi security forces fought pitched battles against Islamic State militants in the city of Mosul in recent months, the country's array of largely Shiite paramilitary groups also has been steadily pushing through the desert west of the city - toward the Syrian border.

Photographs recently circulated showed Maj. Gen. Qasem Soleimani, commander of Iran's elite Quds Force, in the border area with Iraqi forces. Iraq's prime minister Haider al-Abadi, has also visited the popular mobilization forces there and praised their 'achievement” in reaching the border.

Iraqi militias have already sent thousands of fighters to battle in Syria alongside Assad's forces in the vicinity of Aleppo and the Shiite shrine of Sayda Zaineb, just outside Damascus.

'At this stage, it depends less on what Assad and Iran does and more on what the United States does,” said Aron Lund, a fellow at the Washington-based Century Foundation.

'If the U.S. and its allies have built up a strong enough force to move on al-Bukamal, then of course they can get there first. But can they get there in a way that is sustainable? They don't seem to be sure of what they want to do.”

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The Washington Post's Zakaria Zakaria in Istanbul, Mustafa Salim in Baghdad, Dan Lamonthe in Hawaii and Thomas Gibbons-Neff in Washington contributed to this report.