The United States is looking ahead toward a decisive battleground in its bid to destroy the Islamic State group, even as U.S.-backed local forces must still finish the fight for the extremists' two main strongholds in Iraq and Syria.
The roughly 100-mile stretch of ISIS-controlled territory straddling the Iraq-Syria border could represent the start of an endgame for defeating an extremist group that had gobbled up large swaths of territory in the heart of the Middle East, at one point even threatening Baghdad.
Three years into America's campaign, President Donald Trump is studying ways to accelerate the pace. He hasn't yet announced results of a strategy review he ordered the Pentagon to undertake in late January in collaboration with the State Department and other agencies, but officials are already plotting how to wrest the final territories from ISIS' control.
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"ISIS will go down fighting, of course, and do enormous amounts of harm in its death throes," Hal Brands and Peter Feaver, two U.S. defense analysts, wrote in a recent assessment of how the extremist group will be defeated.
Defense Secretary Jim Mattis and Gen. Joseph Dunford, the Joint Chiefs of Staff chairman, are to provide an update on the campaign at a Pentagon news conference Friday. In Europe last week for talks with coalition partners, Mattis said the extremists have lost "well over" half their territory in Iraq and Syria since 2014.
Much fighting remains to fully expel ISIS from Mosul in northern Iraq, and the battle for the extremists' self-declared capital of Raqqa, Syria, has barely begun.
But the follow-on battle lines are already clear. They will be drawn from the Syrian city of Deir el-Zour, which has come under increasing U.S. aerial bombardment, to the western Iraqi city of al-Qaim. The Pentagon refers to this area as the Middle Euphrates River Valley. Islamic State leaders and operatives have gravitated there in apparent anticipate of losing Mosul and Raqqa.
Post-Mosul, post-Raqqa, the intent will be to militarily squeeze this stretch of territory from each end, according to U.S. officials.
American forces would support a group of Syrian Arab and Kurdish fighters known as the Syrian Democratic Forces driving southeast along the Euphrates from Raqqa toward Deir el-Zour, said the officials, who weren't authorized to speak publicly on the military details and demanded anonymity. At the same time, Iraqi government forces, also supported by U.S. advisers and airpower, would advance toward al-Qaim.
And a separate U.S.-backed group of Syrian rebels would push up from the south to block ISIS escape routes, the U.S. officials said.
Deir el-Zour presents an especially tricky challenge because the Syrian military has a base there and the U.S. has avoided tangling with President Bashar Assad's forces other than an April 7 cruise missile strike.
The complexities of the campaign in Syria were illustrated by an unusual U.S. airstrike Thursday against what the Pentagon described as Syrian pro-Assad forces near the Tanf military camp close to the Jordanian border. U.S. special operations forces have been working there with a Syrian opposition group in operations against ISIS.
A U.S. military statement said Thursday's strike occurred after Russia tried unsuccessfully to dissuade pro-Assad forces from advancing toward Tanf.
The overall outlook is further clouded by the Syrian government's announcement this month of a new military push aimed at reasserting its authority in east, including in Deir el-Zour and the remote desert area near Syria's borders with Jordan and Iraq. Syrian Foreign Minister Walid al-Moallem said last week "the main goal" is to reach Deir el-Zour, an oil-producing region that was largely captured by ISIS during its great expansion of territory three years ago.
While the Middle Euphrates River Valley corridor may be the next key battleground, U.S. officials believe it will not be the last. There are other pockets of extremist control in Iraq, including Hawija, west of Kirkuk. The situation is murkier in Syria given the mix of rebel groups involved, some of which occasionally ally with al-Qaida or ISIS. The U.S. also must contend in the country with uncoordinated fighting forces from Turkey, Russia and Iran-backed militia such as Lebanon's Hezbollah.
Military victory against ISIS isn't by itself sufficient. Keeping ISIS or a follow-on extremist group out of liberated areas will likely depend on establishing stable governance and relieving humanitarian crises that have accumulated over years of war in both Syria and Iraq.
"Once we get that threat subdued, then the real work begins," Mattis said this month in Copenhagen. Government services and normalcy must be restored, he said, "otherwise you simply breed the next problem."
In their assessment for the Center for Strategic and Budgetary Assessments, a Washington think tank, defense analysts Brands and Feaver said defeating ISIS militarily won't yield a decisive victory in the broader fight against Islamic extremism.
"So long as the root causes of jihadist ideology persist throughout much of the greater Middle East and beyond, so too will the threat itself," they wrote.