In the late Warring States period, after Qin conquered Wei, the king of Qin sent a messenger to Anling, a vassal lord of Wei, saying, "Our king wishes to exchange five hundred li of land for your fifty li of Anling. Please do not refuse!"
Lord Anling knew this was Qin's scheme to annex his state under a pretext, but dared not offend Qin, so he told the envoy, "The King of Qin offers to exchange large for small—I appreciate his goodwill—but Anling is my ancestral domain, and I dare not trade it away lightly."
When the First Emperor of Qin heard the envoy's report, he was deeply displeased.
Lord Anling knew that the state of Qin would not let the matter rest, and fearing military retaliation, he sent Tang Ju to Qin to express his willingness to foster good relations.
When Emperor Qin Shi Huang received Tang Ju, he demanded, "I offered ten times the land of Anling in exchange—why did Lord Anling refuse? Does he think nothing of me?" Tang Ju replied, "Your Majesty misunderstands. Lord Anling would never dare slight you. He cannot trade away what his ancestors entrusted to him." Enraged, Qin Shi Huang slammed the table and thundered, "Have you heard of an emperor's wrath?" "No," Tang Ju answered calmly. "An emperor's wrath leaves a million corpses and blood flowing for a thousand li!" Tang Ju laughed heartily and shot back, "Have you heard of a commoner's wrath?" The emperor sneered, "A commoner's wrath is just tearing off his hat, baring his feet, and banging his head on the ground!"
Tang Ju replied, "You speak of ordinary anger. When Zhuan Zhu assassinated King Liao of Wu, a comet struck the moon; when Nie Zheng assassinated the prime minister Xia Lei of Wei, a rainbow pierced the sun; when Yao Li assassinated Prince Qing Ji of Wei, a fierce hawk swooped into the palace." Drawing his sword, Tang Ju stepped forward, seized the First Emperor of Qin by the arm, and declared with a murderous glare, "Today, my anger will mean we die together!"
Qin Shi Huang was so terrified that he knelt and begged for mercy, promising never to mention "swapping land" again. Later, the idiom "Blood Flows a Thousand Li" came to describe a scene of war with massive casualties.
Source: *Strategies of the Warring States*, Chapter "Strategies of Wei IV"
Meaning of the Idiom: Later, the Chinese idiom "流血千里" came to describe a scene of war with massive casualties.