During the early Spring and Autumn period, the state of Zheng was once quite powerful and eager to expand its territory.
In 712 BCE, Duke Zhuang of Zheng, using the excuse that the Duke of Xu had disobeyed the Zhou king's orders, allied with the states of Qi and Lu to launch a punitive campaign against Xu.
The allied forces of three states converged beneath the walls of Xu's capital
After capturing Xudu, the three states discussed how to deal with the State of Xu. The Marquis of Qi proposed placing Xu under Lu's jurisdiction, but Duke Yin of Lu declined. The Marquis of Qi then said, "Zheng initiated the campaign and contributed the most. Since Lu cannot accept it, let Xu's territory be ceded to Zheng!"
Duke Zhuang of Zheng's true intention in attacking the State of Xu was to annex it outright, but mindful of appearances, he first went through the motions of declining the territory. He then divided Xu into two parts: the eastern half he entrusted to Baili, a minister of Xu, to assist Xu Shu, the younger brother of the former Duke Zhuang of Xu, in governing; the western half he placed under the control of his own minister Sun Huo, who in reality served as a watchman over Xu Shu in the east.
After making the arrangements, Duke Zhuang of Zheng said to Baili, "Heaven intends to bring disaster upon you, so it uses my hand to punish the State of Xu. Yet I cannot even control my own younger brother (Duke Zhuang's brother, Gongshu Duan, rebelled and fled into exile), leaving him to wander abroad—how could I possibly govern Xu for long?"
Duke Zhuang mentioned his younger brother, feigning benevolence to show he would not keep the State of Xu for himself. Finally, he instructed Baili to properly assist Xu Shu and care for the people of Xu.
The author of *Zuo Zhuan* commented on Duke Zhuang of Zheng's action: "Duke Zhuang of Zheng thus acted with propriety. ... Xu violated the law, so he attacked it; when it submitted, he spared it. He assessed virtue to handle matters, measured strength to act, seized the right moment to move, and did not burden future generations—this can be called understanding propriety."
This passage praises Duke Zhuang of Zheng for attacking Xu for its lawlessness, forgiving it upon surrender, arranging matters according to each person's virtue and conduct, and acting within his own capabilities—all in accordance with "ritual propriety." In reality, this commentary fails to see through Duke Zhuang's feigned modesty and pretense.
Later, people used the idiom "Acting According to One's Ability" to refer to acting within one's own capacity.
Source: *Zuo Zhuan*, Chapter "Duke Yin, Eleventh Year"
Meaning of the Idiom: Later, the Chinese idiom "量力而行" came to describe acting within one's own capacity.