The elite soldiers, who typically operate in the shadows, arrived in armored vehicles festooned in brightly colored American flags, a gesture designed to make their presence abundantly obvious. And unlike the other 15,000 U.S. troops on the ground in active war zones, the Americans in Manbij are not conducting "counter terror" or "advise and assist" operations. 

Rather, the Pentagon has quietly unveiled a new kind of mission: It's called "reassurance and deterrence." 

There are no Islamic State extremists in rubble-strewn Manbij. ISIS fled in defeat many months ago. Instead, the city is occupied by a council of Kurdish militia fighters whom the U.S. has supported for many years. And all around the city are forces backed by the Turkish military, a fellow NATO member that's played a significant role in eradicating ISIS strongholds in Syria. 

The problem is that even though both groups are U.S. allies, the Turks and the Kurds despise each other. Like