WASHINGTON — Last summer, as the fighting in Syria raged and questions about the United States’ inaction grew, Secretary of State Hillary Rodham Clinton conferred privately with David H. Petraeus, the director of the C.I.A. The two officials were joining forces on a plan to arm the Syrian resistance. The idea was to vet the rebel groups and train fighters, who would be supplied with weapons. The plan had risks, but it also offered the potential reward of creating Syrian allies with whom the United States could work, both during the conflict and after President Bashar al-Assad’s eventual removal. Mrs....