Belligerents
Background
Master of Xuzhou, Lü Bu had driven out Liu Bei and cycled through alliance and rupture with Yuan Shu, menacing Cao Cao’s eastern flank. When the dispossessed Liu Bei took refuge with Cao Cao, the two joined hands against their common enemy, and in 198 Cao Cao marched on Xuzhou in force.
Course
Beaten in the field, Lü Bu holed up in Xiapi; before Chen Gong’s plan for an outside-inside pincer could be tried, Cao Cao diverted two rivers and drowned the city’s approaches. Three months of flood broke the garrison’s spirit, and Lü Bu’s own officers bound Chen Gong and opened the gates. The captured Lü Bu bargained — "with me you could take the realm" — until Liu Bei’s quiet reminder of Ding Yuan and Dong Zhuo sealed the execution. Chen Gong walked to the block himself; Zhang Liao entered Cao Cao’s service.
Outcome & impact
The age’s mightiest warrior left the stage, and Cao Cao, with Xuzhou secured, could turn to the reckoning with Yuan Shao. Gaining the future great general Zhang Liao was no small bonus. The cohabitation of Cao Cao and Liu Bei, of course, did not last.
History vs. the novelHistoryvsNovel
History and novel largely agree here. The White Gate Tower theatrics — Lü Bu’s dying curse on Liu Bei as "the least trustworthy of men" — are the novel’s polish; the historical execution scene is terser.