Genghis: Lords of the Bow by Conn Iggulden

The gathering of the tribes of the Mongols has been a long time in coming, but finally, triumphantly, Temujin of the Wolves, Genghis Khan, is given the full accolade of overall leader and their oaths. Now he can begin to meld all the previously warring people into one army, one nation. But the task Genghis has set himself, and them, is formidable. He is determined to travel to the land of the long-time enemy, the Chin, and attack them there.