What is the fine-grained topic of the following text: The Han's senior minister, Cao Cao, has rescued Emperor Xian from Dong Zhuo's minions, established a temporary capital in Xuchang, quelled most of the rebels and dealt a crippling defeat to his most powerful rival, Yuan Shao, at the Battle of Guandu.The loss at Guandu left Yuan Shao a sick man, and thoughts turn t… o his successor: the resultant crisis begins another era of chaos that empowers the Wuhuan tribes of the northeast and threatens to undo all that has been achieved. Only return the topic.
The historical events and political turmoil during Cao Cao's leadership in the Han Dynasty.