Last April, a 62-year-old American man walked across the border from Mexico back to the United States. He carried four giant wheels of cheese. Border agents stopped him. He told them that he simply wanted good Mexican cheese to make quesadillas. But he was actually attempting to smuggle $13,000 worth of methamphetamine into the U.S. That was just one of a record number of methamphetamine busts at the border last year. How did Mexico become the country’s biggest meth source?