During the Spring and Autumn period, the Zhou king's authority was weak, and the land was divided into many states, with powerful ones like Qin and Jin vying for dominance by expanding their territories.
Actually, the states of Qin and Jin had long maintained close ties, often sealing their friendship through marriage alliances. For instance, Duke Mu of Qin's wife was the daughter of Duke Xian of Jin, and Duke Mu of Qin helped stabilize Jin's throne three times. When Prince Chong'er of Jin—later known as Duke Wen of Jin—was forced into exile, it was with Qin's substantial assistance that he returned to claim his rightful place as ruler.
However, because the two kingdoms shared a border and each sought to expand its territory, their otherwise friendly relations were frequently marred by conflicts over spheres of influence. For three generations, from Duke Mu of Qin to Duke Huan of Qin, the states of Qin and Jin were locked in years of continuous warfare.
After Duke Li of Jin ascended the throne, disputes over border issues arose between the states of Qin and Jin, prompting the two rulers to meet at Linghu (west of present-day Linyi, Shanxi) to sign a treaty and resolve the conflict.
However, after Duke Huan of Qin returned to his state, he immediately betrayed the alliance. He invited the state of Jin to join him in attacking a small border state called Baidi. Baidi was an enemy of Qin but had marriage ties with Jin. Fearing Qin's power, Jin had no choice but to agree.
But at this time, the state of Qin sent someone to the state of Baidi, saying, "Jin is about to attack you! You should submit to us, Qin, and join us against Jin."
When the ruler of the Jin State learned of Qin's treacherous breach of faith, he was deeply outraged and sent Lv Xiang to sever ties with Qin. Lv Xiang declared to Duke Huan of Qin, "Every state now knows that Qin is a kingdom driven solely by profit and utterly devoid of honor. Thus, all prefer to maintain friendly relations with Jin while viewing Qin with bitter resentment. We and the allied states are fully prepared for war. If Qin is willing to honor the covenant, Jin will persuade the allies to withdraw. Otherwise, we shall meet on the battlefield."
Duke Huan of Qin showed no regard for the state of Jin, so Jin led a coalition of feudal lords to battle against Qin at Masui, where the Qin army suffered a crushing defeat.
Later, the idiom "Painful Heart and Aching Head" came to describe extreme sorrow and hatred.
Source: *Zuo Zhuan*, "Duke Cheng's Thirteenth Year"
Meaning of the Idiom: Later, the Chinese idiom "痛心疾首" came to describe extreme sorrow and hatred.