During the Warring States period, as the state of Chu grew increasingly weak, King Xiang of Chu placed his trust in four flattering but incompetent ministers: Marquis Zhou, Marquis Xiahou, Lord Yiling, and Lord Shouling.
The minister Zhuang Xin, seeing King Xiang of Chu so confused, submitted a memorial to remonstrate:
"Zhou Hou, Xia Hou, *Ling Jun, and Shou Ling Jun only seek comfort and pleasure, ignoring the rise and fall of the state. In the future, Yingdu (the capital of Chu) will surely face danger." King Xiang, however, disagreed and mocked Zhuang Xin, saying, "You are old, sir, perhaps a bit confused. Do you think some monster has appeared in Chu?"
Zhuang Xin said gravely and earnestly, "I dare not regard these four as monsters, but I am certain that if Your Majesty continues to favor them, the state will be in grave danger. I request permission to take temporary refuge in the State of Zhao."
King Xiang nodded in agreement, and Zhuang Xin then departed for the state of Zhao.
Five months later, Zhuang Xin's grim prophecy came true. The Qin army launched a massive invasion of Chu, capturing vast territories including the capital Ying. King Xiang was forced to flee the city and found himself trapped in Chengyang (present-day Cheng, Shandong).
King Xiang, trapped in a difficult situation, deeply regretted not heeding Zhuang Xin's advice earlier, so he sent someone to Zhao to summon him back. After Zhuang Xin returned to Chengyang, the king said to him shamefully, "I failed to take your counsel seriously before, leading to this disastrous defeat. What do you think I should do now?"
Zhuang Xin said, "As the saying goes, 'It's not too late to unleash the hounds once you spot the rabbit, and it's not too late to mend the pen after losing the sheep.' Though Chu has suffered a heavy blow, our strength remains considerable, with thousands of li of territory. If the people unite and strive, we can rally and rise again."
After hearing Zhuang Xin's counsel, King Xiang of Chu felt deeply convinced and his spirits lifted. He appointed Zhuang Xin as Lord of Yangling and, leveraging his wisdom and talent, launched a counterattack against the Qin army, reclaiming much lost territory.
Later, people used the idiom "mending the pen after losing a sheep" to describe that it is not too late to remedy a mistake or failure in time.
Source: *Strategies of the Warring States*, Chapter "Strategies of Chu IV"
Meaning of the Idiom: Later, the Chinese idiom "亡羊补牢" came to describe it is not too late to remedy a mistake or failure in time.