Daraa is the fourth and most recent area of Syria covered by “de-escalation agreements” — a series of cease-fire arrangements brokered since May by the U.S., Syria’s Sunni Muslim neighbors Jordan and Turkey, Iran’s Shiite Islamic Republic, Syrian President Bashar Assad and his Russian backers. Mr. Trump has repeatedly pointed to the cease-fire accord he and Mr. Putin reached in their lengthy G-20 meeting, in part to bolster his argument that engaging with Moscow can deliver real-world results despite the deep doubts of the Washington foreign policy establishment.