Seventy years ago, almost to the day, a group of American ambulance drivers, disgusted by the waste and carnage they’d seen in the World Wars in Europe, started up a cultural exchange program between Europe and the U.S. The idea was simple: If people knew each other—really knew each other—they would be more tolerant of one other. It’s harder to demonize someone once you’ve stayed in their homes and shared meals and stories together.