During the mid-Tang Dynasty, after the An Lushan Rebellion, the central government's power weakened significantly, and regional military governors, known as jiedushi, grew increasingly independent, ignoring imperial commands—none more so than the governor of Huaixi.
In 814 AD, the Huaixi military governor Wu Shaoyang died, and his son Wu Yuanji, failing to inherit his father's post, took command of the army and launched a rebellion. Despite multiple imperial campaigns to suppress him, all ended in failure, making Wu Yuanji even more arrogant.
In 816 AD, Chancellor Pei Du was leading a campaign against the rebel Wu Yuanji. Recognizing General Li Su as both brave and brilliant, Pei Du appointed him field commander.
Li Su lived up to expectations, tirelessly drilling his troops, tightening discipline, and boosting morale, leading by example in every way and caring for his soldiers' hardships, which greatly enhanced the army's combat effectiveness.
After six months of preparation, General Li Su led his army toward Huaixi, winning several battles in a row and greatly boosting morale. In one engagement, Li Su captured Wu Yuanji's chief advisor, Wu Xiulin. Li Su explained the greater cause and persuaded Wu Xiulin to surrender to the imperial court, who then helped Li Su campaign against Wu Yuanji.
The following winter, on a bitter, snowy night, General Li Su led a light cavalry force on a daring 130-li night march, catching the rebel Wu Yuanji completely off guard at his stronghold in Caizhou, and swiftly stormed the city before the enemy could mount a defense.
Wu Yuanji was jolted from his sleep to find Li Su's troops surrounding his residence. As he scrambled to climb the wall and escape, Tang soldiers captured him alive.
With their commander captured, the remaining forces in Huaixi surrendered one after another, and the rebellion was swiftly and completely pacified.
When the local people saw the imperial army had pacified Caizhou, they clasped their hands to their foreheads in celebration. Having suffered greatly under Wu Yuanji's tyranny, they wept tears of gratitude toward the army that had reclaimed their city.
At the time, Tang Dynasty poet Liu Yuxi, upon hearing the news of Caizhou's recapture, was deeply moved and immediately wrote three poems titled "Ping Cai Xing" to celebrate. The line in the second poem, "An old man by the roadside recalls past events, and they all weep with gratitude," vividly reflects the people's emotional scene at that moment.
Later, the idiom "grateful tears streaming down" came to describe a state of being deeply moved.
Source: Liu Yuxi (Tang Dynasty), *Ping Cai Xing*
Meaning of the Idiom: Later, the Chinese idiom "感激涕零" came to describe a state of being deeply moved.