Erik Prince, the founder of Blackwater, has a new project. ABU DHABI, United Arab Emirates — Late one night last November, a plane carrying dozens of Colombian men touched down in this glittering seaside capital. Whisked through customs by an Emirati intelligence officer, the group boarded an unmarked bus and drove roughly 20 miles to a windswept military complex in the desert sand. The Colombians had entered the United Arab Emirates posing as construction workers. In fact, they were soldiers for a secret American-led mercenary army being built by Erik Prince, the billionaire founder of Blackwater Worldwide, with $529 million...