In 354 BCE, King Wei Hui (魏惠王 wèi huì wáng) of the Wei Kingdom (魏国 wèi guó), could not let go of the fact that he had yielded Zhong Shan (中山 zhōng shān), so he sent General Pang Juan (大将庞涓 dà jiàng páng juān) to take it back.
Zhong Shan was a small country adjacent to Wei during the Eastern Zhou Dynasty (东周 dōng zhōu) (770–256 BCE). It was conquered by Wei many years prior, however, when Wei was going through difficult times, the Zhao Kingdom (赵国 zhào guó) took the opportunity to occupy Zhong Shan.
But General Pang Juan believed that Zhong Shan was insignificant compared to Zhao itself. Why not just attack Zhao's capital, Han Dan (邯郸 hán dān), and take them all in one fell swoop? King Wei Hui agreed, and so the battle began. He provided General Pang Juan with an army and five hundred horse-driven chariots and within no time, they had surrounded Han Dan.
In desperation, the King of Zhao had no choice but to request assistance from the Kingdom of Qi (齐国 qí guó), promising to give them Zhong Shan if they could help them out of their predicament. The King of Qi agreed and immediately selected a general, Tian Ji (田忌 tián jì), who subsequently chose Sun Bin (孙膑 sūn bìn) to lead their army.
Sun Bin used to live in Wei, and was once a fellow classmate of General Pang Juan. Sun Bin excelled in his studies, so Pang Juan - fearing that Sun Bin would surpass him - poisoned Sun Bin, cut off his feet, and tattoed his face, in an effort to have him cast aside by society. It worked. Sun Bin later escaped to Qi and was now his rival.