Complex sentence: Cao Cao became the next hegemonist to seize control of the Emperor as Dong Zhuo, Li and Guo had done before him, and the power of Li, Guo, and Zhang greatly diminished resulting from previous infighting and the loss of Emperor Xian.
Split into simpler sentences: Cao Cao became the next hegemonist to seize control of the Emperor as Dong Zhuo, Li and Guo had done before him. The power of Li and Guo had greatly diminished resulting from previous infighting and the loss of Emperor Xian.