DAMASCUS, Aug. 26 -- The announcement was brief. Mazloum Abdi, the Kurdish commander who built the Syrian Democratic Forces into the principal ground partner of the American campaign against the Islamic State, stood before reporters on Monday and confirmed what months of negotiation had been building toward: the SDF would dissolve, its fighters integrating into the Syrian national army under Damascus's authority.

By Tuesday, the US Special Envoy for Syria called the outcome historic.

Tom Barrack, appointed by the Trump administration to manage Washington's Syria policy after the fall of the Assad government, hailed the dissolution as evidence that a "united Syria for all takes shape." The statement framed the development as the fulfillm...