Qing Zheng, a strategist under Duke Hui of Jin during the Spring and Autumn period, watched as Qin forces invaded Jin, winning three consecutive battles and charging into Jin's territory, leaving the state in dire straits.
Duke Hui of Jin resolved to personally lead the campaign against the Qin army, so he ordered his chariot to be harnessed with famous horses from Zheng, which were tall and strong. He believed using these horses would benefit the battle. Qing Zheng, who was nearby, learned of this and advised him:
"In ancient times, for sacrificial rites or major battles, horses from one's own country were always used, because they grew up on native soil, were accustomed to local conditions, understood the people's hearts, obeyed their commands, and knew the local roads—so when harnessed to a chariot, they would never disobey your will."
Duke Hui of Jin heard this but disagreed. Qing Zheng continued,
"Now you've switched to a horse from the state of Zheng, but you don't know its temperament—this is far too dangerous," the advisor warned. "This horse is tall and powerful, but once startled, it becomes impossible to control. In its panic, its blood vessels swell and its breathing turns rapid. Though it looks strong on the outside, inside it's already exhausted. If something goes wrong, the consequences would be unthinkable—you'd be unable to advance or retreat, and regret would come too late."
However, Duke Hui of Jin still refused to heed Qing Zheng's advice, stubbornly insisting on yoking the horses from Zheng to his chariot and setting off to battle.
Soon after, the armies of Qin and Jin clashed fiercely at Hancheng. In the heat of battle, Duke Hui of Jin's chariot horses became mired in thick mud. Panicked, the horses reared and struggled wildly, only sinking deeper until they were completely stuck, unable to move forward or back. This disastrous turn led to a crushing defeat for Jin, and Duke Hui himself was captured by Qin.
Later, the idiom "outwardly strong but inwardly weak" came to describe something that appears powerful but is actually fragile.
Source: *Zuo Zhuan*, "The Fifteenth Year of Duke Xi"
Meaning of the Idiom: Later, the Chinese idiom "外强中干" came to describe how something that appears powerful is actually fragile.