‘Were you able to sew it back on?” That was the question a 9-year-old Syrian boy asked me when he woke from surgery. His hand was obliterated after a helicopter dropped a barrel bomb on his school. All I could do was clean his wound, wrap it with gauze, and tell him he was going to be all right. Deep inside I knew I couldn’t even promise him that. I still saw helicopters passing outside. Life in a Syrian hospital means anticipating death at every moment.

Barrel bombs are metal cylinders and drums packed with explosives and...