The Mongol army under Genghis Khan, generals and his sons crossed the Tien Shan mountains by entering the area controlled by the Khwarezmian Empire. After compiling intelligence from many sources Genghis Khan carefully prepared his army, which was divided into three groups. His son Jochi led the first division into the northeast of Khwarezmia. The second division under Jebe marched secretly to the southeast part of Khwarzemia to form, with the first division, a pincer attack on Samarkand. The third division under Genghis Khan and Tolui marched to the northwest and attacked Khwarzemia from that direction.