Battle of Guandu
Cao Cao defeated Yuan Shao, the most powerful warlord in the north, at Guandu, opening the way to his unification of northern China.
In 196 Cao Cao brought Emperor Xian to Xu and began issuing orders in the emperor’s name. When Yuan Shao marched south with a large army in 200, Cao Cao held at Guandu with a smaller force, then raided and burned Yuan Shao’s grain depot at Wuchao. Yuan Shao’s army collapsed and he died in 202. Cao Cao went on to defeat Yuan Shao’s sons one by one and controlled the north by 207. The battle is a classic example of a smaller army beating a larger one.