By the end of the 190s, northern China was divided between two contending warlords, Yuan Shao and Cao Cao. Each had assimilated the smaller powers around them and made political arrangements with the larger powers elsewhere, and were increasingly looking at each other for potential gains. Cao Cao had earlier secured the helpless Emperor Xian into his custody, a move that Yuan Shao regretted not to have made. After the successful conquest of Gongsun Zan's territories to the north, Yuan Shao turned to the south to prepare for an assault on Cao Cao in the summer of 199. In response, Cao Cao went on a patrol to the northern front and dispatched several armies to defend positions on the south side of the Yellow River in the eighth lunar month (September 8 - October 7) of 199. Among these was th