The other pressures on the Kurdish areas are immense, and mounting. In response to the U.S. announcement of the border force, Turkish President Recep Tayyip Erdogan promised to “destroy all terror nests one by one in Syria.” Erdogan has sent tanks and troops to the border with a vow to wipe out the Syrian city of Manbij, where American Special Operations Forces train fighters and maintain outposts. In a speech on Monday, Erdogan threatened American troops directly, warning, “don't stand between us and these herd of murderers. Otherwise, we won't be responsible for the unwanted incidents that may arise.”

Beyond the military strategy and its risks, it’s hard to get details about the U.S.-sponsored reconstruction work in Syria. One State Department official who spoke on the condition of anonymity acknowledged that more U.S. aid workers and diplomats are heading for the Kurdish-dominated northeast this year, and they plan to spend approximately $400 million on economic assistance and stability work. The money might seem a relatively small investment compared to the hundreds of billions Washington has spent in Iraq and Afghanistan on reconstruction over the past 16 years, but it does raise the question of what exactly the spending supports.

“We have now entered the stabilization phase of the Trump strategy in Syria, but it remains unclear what that strategy is,” said Nicholas Heras, of the Center for New American Security who studies the Syrian conflict. “The rebuilding in areas the SDF has recaptured like Aleppo and Manbij has been quite slow, with a lack of funding from both the U.S. and international community. … Donors from around the world want guarantees that the U.S. will stay on the ground in Syria, and they won’t have to work through Damascus—and the longer the American position remains unclear, the more unstable some of these devastated areas may become.”

In Raqqa and Tabqa, cities in SDF-controlled areas, U.S. teams are working with locals to facilitate humanitarian assistance, direct de-mining activities, and “prioritize and implement stabilization activities,” a diplomatic official said. The Americans are working particularly closely with the Raqqa Civil Council to get humanitarian aid to tens of thousands of displaced civilians, and U.S. Army engineers recently built a new, permanent bridge just outside of the city in order to help move more supplies in. Another project that has about 80 Western contractors already in place is a $41 million State Department program to clear unexploded ordnance and defuse booby-trapped buildings left behind after ISIS was pushed out.

“Right now the key foreign policy interest is stabilizing these areas and creating a sense of hope in these communities that were brutalized by ISIS,” said Stan Brown, director of the office of Weapons Removal and Abatement at the State Department. Brown’s office hired the contractors to do the de-mining work, and has trained roughly 120 locals to help with the monumental task. He said the sheer volume of explosives left behind in northern Syria will take years to clear.