Genghis Khan has united the warring tribes and even taken his armies against the great cities of their oldest enemies. Now he finds trouble rising west of the Mongolian plains. His emissaries are mutilated or killed; his trading gestures rebuffed. So, dividing his armies, using his sons as generals of the various divisions, he sends them out simultaneously in many directions, ranging as far as modern Iran and Iraq. As well as discovering new territories, exacting tribute from conquered peoples, laying waste the cities which resist, this policy is also a way of diffusing the rivalries between his sons and heirs and working out who should succeed the khan.