Too poor to even buy pens and notebooks for school, Mehdi left his home in Afghanistan soon after his 17th birthday and headed to Iran, hoping to make his way to Europe and find work. Instead, Mehdi ended up fighting in Syria’s civil war, a conflict he had nothing to do with, 2,000 kilometers (1,200 miles) from home. He was one of tens of thousands of Afghans recruited, paid and trained by Iran to fight in support of Tehran’s ally, Syrian President Bashar Assad. There, he found himself thrown into one of the war’s bloodiest front lines, surrounded by the...