In the second half of 2017, the US-backed Syrian Democratic Forces (SDF) stormed down the east bank of the Euphrates taking village after village from the Islamic State (IS) while the Syrian Arab Army (SAA) and its allies liberated besieged Deir Ez-Zor and the river’s western bank. IS now holds but a few barren pockets of land (see map).

Though largely arid and unpopulated, the lands by the east bank of the Euphrates contain some of Syria’s largest oil fields, which were...