during the spring and autumn period (770-476 b.c.), duke xian of the state of jin killed the crown prince sheng because he had heard slanders1 about sheng and believed them. he also sent his men to arrest chong er, shen sheng's brother. hearing the news, chong er escaped from the state of jin, remaining a fugitive2 for more than ten years.

after innumerable hardships, chong er arrived at the state of chu at last. king cheng of the state of chu treated him with high respect as he would have treated the ruler of a state, believing that he would have a bright future.

one day, king cheng of the state of chu gave a banquet in honoudr of chong er. suddenly, amid the harmonious3（和谐的） atmosphere of drinking and talking, king cheng of the state of chu asked chong er. "how will you repay me when you return to the state of jin and become its ruler one day?" after thinking for a moment, chong er said, "you have plenty of beauties and attendants as well as jewelry4 and silk cloth, and the state of chu abounds5 in rare brides and animals. what treasure can the state of jin boast having to present to your majesty6?" king cheng of the state of chu said, "you are too modest. nevertheless, you still have to show your gratitude7 to me in one way or another, i presume?" smiling, chong er answered, "if i should be fortunate enouge to return to the state of jin and become its ruler, the state of jin would be friendly to the state of chu. if, one day, there should be a war between the two states, i would definitely order my troops to retreat three she (one she is equivalent to thirty li. the li is a chinese unit of length equivalent to 1/2 kilometre. and, therefore, three she is about thirty miles.) as a condition for peace. if, under that condition, you were still not reconciled, i would have to fight with you."

four years later, as might be expected, chong er returned to the state of jin and became its ruler. he was none other than（不是别的而正是） duke wen of the state of jin famous in ancient chinese history. ruled by him, the state of jin became increasingly powerful.

in the year 633 b.c., the chu troops and the jin troops confronted each other in a battle. faithful to his promise, duke wen of the state of jin ordered his troops to retreat about thirty miles. after retreating, the jin troops were stationed at chengpu. seeing that the jin troops were retreating, the chu troops thought that the enemy troops were afraid, and began chasing them. taking advantage of the chu troops' arrogance8 and their talking the jin troops lightly, the jin troops concentrated their forces and inflicted9 a crushing defeat on the chu troops, thus winning the victory of the battle of chengpu.

this set phrase, "retreating about thirty miles as a condition for peace," is derived10 from the chapter "the twenty-second year of duke xi" in zuozhuan, the famous commentary by zuo qiuming on the spring and autumn annals. the idea of this set phrase is to give way to somebody in order to avoid a conflict.