The Battle of Guandu was a major battle between Han Dynasty-connected lords Cao Cao and Yuan Shao. Cao Cao won a decisive victory, and Yuan Shao was forced into exile. As a result, Cao Cao became the effective ruler of Northern China.
By 201, Cao Cao and Yuan Shao were the most powerful men in the dynasty. As Prime Minister, Cao Cao commanded the authority of the Emperor. The prideful Yuan Shao, worried that Cao Cao may attempt to overthrow the Emperor, gathered a force of 100,000 at the border of his territory, attempting to intimidate Cao Cao. In response, Cao Cao gathered his own force and launched a preemptive attack against Yuan Shao.
Cao Cao's army was much smaller than Yuan Shao's (30,000-60,000 depending on the source); never the less, Cao Cao made the first attack. He sent small forces of 1,000 men each to attack cities along the Yellow River. Worried that Cao Cao may have a larger army than it appeared, Yuan Shao shifted his troops to secure the river. As a result, his defenses were spread thinly.
In summer, Yuan Shao assembled a force of horseman to capture Boma, a forward base on Cao Cao's side of the Yellow River. As he had very few horseman himself, Cao Cao did not attempt to secure the base. He left a small force in the base and sent his troops north to attack Liyang, a city in Yuan Shao's side of the river. Hearing of the attack, Yuan Shao panicked and tried to call back his horsemen. Before they could return, Cao Cao's army withdrew to the Yellow River. This confused Yuan Shao, who was unsure if he should attack again.
By winter, Yuan Shao's forces were in disarray. Cao Cao's advisors, led by Xu You, advised Cao Cao to hold back and wait until Yuan Shao's supplies ran low. Cao Cao ignored these requests and moved decisively to invade Yuan Shao's territory. Crossing the river at Yuanwu, his force marched on Ye, Yuan Shao's home city. As Yuan Shao's forces were spread across the border, Cao Cao easily broke through the defenses. As he moved, he set fire to supply bases, leaving a burned trail. Yuan Shao, crippled by indecision, left his officers to fend for themselves while he and his advisors tried to come up with a plan. By the beginning of 202, Ye had fallen. Cao Cao sent Yuan Shao into exile and claimed his territory for himself.