Cao Cao led an army of 200,000 troops and marched to Xuzhou, intending to get rid of Liu Bei before he built his forces. Liu Bei was defeated and sought shelter with Yuan Shao in Yizhou.
Guan Yu was sent to Xiapi to protect Liu Bei's family. After Cao Cao and his troops managed to encircle Xiapi, he sent Zhang Liao to convince Guan Yu into surrendering.
Guan Yu agreed to surrender under 2 conditions - he'd only submit to the Han Emperor and that he be allowed to rejoin Liu Bei later. Cao Cao accepted Guan Yu's conditions, and Guan Yu temporarily submitted to Cao Cao and followed his troops back to Xuzhou.
In order to win Guan Yu over, Cao Cao showered him with beautiful women and presents. However, Guan Yu gave all the gifts to his sisters-in-law. Cao Cao even made a new robe for Guan Yu, but that too, was rejected. Finally, Cao Cao presented him with a Red Hare, to which Guan Yu happily accepted.
The following year, Yuan Shao and Liu Bei led their troops into battle with Cao Cao. Yuan Shao's general, Yan Liang, slew 2 of Cao Cao's generals in succession. When Cao Cao requested Guan Yu to break the enemy's defence, Guan Yu killed 2 of Yuan Shao's generals.
This infuriated Yuan Shao. When he confronted Liu Bei and accused Guan Yu of helping the enemy, Liu Bei promised he'd write to Guan Yu and summon him back.
Upon receiving Liu Bei's letter, Guan Yu set all the treasures from Cao Cao on the table and escorted his sisters-in-law in search of Liu Bei. Cao Cao came after Guan Yu and gave him a fighting robe as a gift...to which Guan Yu accepted.
Guan Yu then forced his way through 5 passes and killed 6 of Cao Cao's guarding generals. After Liu Bei was reunited with Guan Yu and Zhang Fei, they gathered their troops in Runan and prepared for battle with Cao Cao once more.
- Compiled from The Three Kingdoms