The past two decades have exhausted Americans’ willingness to accept or even risk extended military action in the Middle East. Whoever takes the oath as president in January 2021 will face a daunting task: building a sustainable Middle East policy on the ruins of U.S. engagement in the region.

The events of recent weeks have revealed the weakness of the U.S. position. Bashar Assad’s regime, now a wholly owned subsidiary of Khamenei & Putin Inc., is pursuing a ruthless offensive in Idlib, Syria’s last rebel-held province....