A hundred years before Columbus and his fellow Europeans began making their way to the New World, fleets of giant Chinese junks commanded by the eunuch admiral Zheng He and filled with the empire's finest porcelains, lacquerware, and silk ventured to the edge of the world's "four corners." It was a time of exploration and conquest, but it ended in a retrenchment so complete that less than a century later, it was a crime to go to sea in a multimasted...