During a White House briefing early last month, the C.I.A. director, Mike Pompeo, recommended to President Trump that he shut down a four-year-old effort to arm and train Syrian rebels. The president swiftly ended the program. The rebel army was by then a shell, hollowed out by more than a year of bombing by Russian planes and confined to ever-shrinking patches of Syria that government troops had not reconquered. Critics in Congress had complained for years about the costs — more than $1 billion over the life of the program — and reports that some of the C.I.A.-supplied weapons had...