Sam Monroe is the reluctant commander of a tough-minded warrior people living in what was once northern Mexico. His tiny country is flanked on the northeast by the Kingdom River, a vast, trade-driven nation that replaced the southern United States, and on the northwest by the Khanate, an empire of nomads who swept down the west coast after crossing the ice from what was once Russia. Sam's people cling to a precarious, hard-won freedom. Toghrul Khan,...