During the early Warring States period, Zhi Bo, a powerful minister of the Jin state, after leading the Han, Zhao, and Wei clans to destroy the Zhonghang clan, demanded territory from the Han, Zhao, and Wei. Fearing Zhi Bo, the Han and Wei clans each ceded land, but Zhao Xiangzi refused. Enraged, Zhi Bo rallied the armies of Han and Wei to attack Zhao.
Zhao Xiangzi, following the strategy of his advisor Zhang Mengtan, secretly contacted the states of Han and Wei, explaining that if Zhao were destroyed, it would be detrimental to them as well. Eventually, the armies of Han, Wei, and Zhao secretly united, launched a surprise attack on the forces of Zhi Bo, and captured him alive.
After the conflict, Zhang Mengtan had rendered great service to the state of Zhao. Yet once victory was achieved, he submitted his resignation to Zhao Xiangzi, declaring he would no longer serve.
Zhao Xiangzi found this very strange and asked Zhang Mengtan why he insisted on retiring after achieving success. Zhang Mengtan replied, "An ancient saying goes: the reason the Five Hegemons of the Spring and Autumn period could govern the realm so well was that their rulers always controlled their ministers, never the other way around. Now I, as a minister, have become too famous, too highly ranked, too powerful, and have won over too many followers. It is time for me to abandon my achievements and shed my authority."
Zhao Xiangzi said with displeasure, "I have heard that those who can assist a ruler achieve fame, those with great merit attain high rank, those responsible for the state are entrusted with important tasks, and those who are loyal earn the trust of all. This is how the ancient sages brought peace to the kingdom. Why do you insist on leaving?"
Zhang Moutan replied, "Your Majesty speaks of what is necessary for success, but I speak of the principles for securing the state. I have heard that in ancient times, rulers and ministers often achieved great victories together, yet after success, when their power became equal, no minister ever met a good end. Remembering the past guides the future. Even if you refuse my resignation, I no longer have the strength to serve you."
Seeing how determined he was, Zhao Xiangzi knew there was no persuading him to stay and reluctantly agreed to let him go.
Later, the idiom "Past experience, if not forgotten, is a guide for the future" came to describe learning from past lessons to guide future actions.
Source: *Strategies of the Warring States*, Chapter "Strategy One of Zhao"
Meaning of the Idiom: Later, the Chinese idiom "前事不忘，后事之师" came to describe learning from past lessons to guide future actions.