Genghis: Lords of the Bow (Conqueror #2) by Conn Iggulden

He came from over the horizon, a single Mongol warrior surrounded by his brothers, sons, and fellow tribesmen. With each battle his legend grew and the ranks of his horsemen swelled, as did his ambition. For centuries, primitive tribes had warred with one another. Now, under Genghis Khan, they have united as one nation, setting their sights on a common enemy: the great, slumbering walled empire of the Chin.