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. 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 ended up in the hands of a rebel group tied to Al Qaeda further sapped political support for the program. President Barack Obama ... agreed to the program in 2013. It soon fell victim to the constantly shifting alliances in Syria's six-year-old civil war. Once C.I.A.-trained fighters crossed into Syria, C.I.A. officers had difficulty controlling them. The fact that some of their C.I.A. weapons ended up with Nusra Front fighters - and that some of the rebels joined the group - confirmed the fears of many in the Obama administration when the program began. Although the Nusra Front was widely seen as an effective fighting force against [President Bashar al-Assad]'s troops, its Qaeda affiliation made it impossible for the Obama administration to provide direct support for the group. American intelligence officials estimate that the Nusra Front now has as many as 20,000 fighters in Syria, making it Al Qaeda's largest affiliate. Officials also received ... reports that the C.I.A.-trained rebels had summarily executed prisoners and committed other violations of the rules of armed conflict.