During the Spring and Autumn Period (770-476 BC), Duke Xian of Jin executed his crown prince, Sheng, after believing slanders against him. He then sent men to arrest Sheng's brother, Chong Er. Upon hearing this, Chong Er fled Jin and remained a fugitive for over a decade.
After enduring many hardships, Chong Er finally reached the state of Chu. King Cheng of Chu treated him with the respect due to a ruler, believing he had a promising future.
One day, King Cheng held a banquet in Chong Er's honor. Amidst the harmonious atmosphere, the king asked, "How will you repay me if you one day return to Jin and become its ruler?"
After a moment's thought, Chong Er replied, "Your Majesty already possesses abundant beauties, attendants, jewels, and silks. Chu is rich in rare birds and animals. What treasure could Jin possibly offer?"
"You are too modest," said King Cheng. "Surely you must express your gratitude in some way?"
Smiling, Chong Er answered, "Should I be fortunate enough to rule Jin, I will ensure Jin's friendship with Chu. However, if war ever breaks out between us, I will order my troops to retreat three she (about thirty miles) as a gesture for peace. Only if you still press forward would I have no choice but to fight."
Four years later, Chong Er returned to Jin and became its ruler—the historically famous Duke Wen of Jin. Under his rule, Jin grew powerful.
In 633 BC, the armies of Chu and Jin met in battle. True to his word, Duke Wen ordered his troops to retreat thirty miles and encamp at Chengpu. The Chu army, mistaking the retreat for fear, pursued them arrogantly. Seizing this advantage, the Jin forces concentrated their strength and inflicted a crushing defeat on the Chu troops at the Battle of Chengpu.
The idiom "retreating thirty miles" originates from Zuo Zhuan, a commentary on the Spring and Autumn Annals. It conveys the idea of making a concession to avoid conflict.