During the Warring States period, Lord Mengchang, a noble of Qi named Tian Wen, was granted the fief of Xueyi by the king. When the powerful state of Chu launched a major attack on Xueyi, Lord Mengchang sent troops to defend while urgently requesting reinforcements from King Xuan of Qi.
At that moment, a Qi court official named Chunyu Kun was returning from a diplomatic mission to Chu and happened to pass through Xue. When Lord Mengchang heard of Chunyu Kun's arrival, he was overjoyed and personally went to greet him, escorting him back to his residence.
Lord Mengchang said warmly to Chunyu Kun, "The Chu army is now attacking my small Xue city, and it's about to fall. I hope you can ask the King of Qi for support when you return, or else Tian Wen fears he will never have the chance to host you here again."
"Rest assured, once I return to the capital, I will certainly plead your case before the king," Chunyu Kun reassured Lord Mengchang.
The military situation was urgent, so Chunyu Kun rested only briefly in Xueyi before racing without pause back to the capital city of Linzi.
King Qi saw Chunyu Kun return from his mission and asked, "What did you see in Chu?"
Chunyu Kun cleverly replied, "The people of Chu are somewhat unreasonable, while our Lord Mengchang is a bit overconfident. Regardless of whether his strength can defend his territory, he built the ancestral temple of the former king there. Now the Chu army is fiercely attacking Xue, and the temple of the former king will be destroyed sooner or later."
Upon hearing this, King Xuan of Qi turned pale with shock and exclaimed, "So the ancestral temple of the former king is in Xueyi! That place must not fall!"
He immediately dispatched reinforcements, who marched through the night to Xueyi
Later, people used the idiom "Overestimating One's Strength" to refer to failing to realistically assess one's own abilities.
Source: *Strategies of the Warring States*, Chapter "Strategies of Qi III"
Meaning of the Idiom: Later, the Chinese idiom "不自量力" came to describe failing to realistically assess one's own abilities.