On a recent Tuesday evening in London, the surgeon David Nott attended a dinner at Bluebird, an upscale Chelsea restaurant. The room was packed with doctors, renowned specialists who had come for the annual consultants’ dinner of the Chelsea and Westminster Hospital, one of Britain’s leading medical establishments. As waiters set down plates of lamb and risotto, Nott checked his phone and found a series of text messages. “Hi David,” it began. “This is an urgent consultation from inside Syria.” Attached was a photograph of a man who had been shot in the throat and the stomach.