The Battle of Jinyang (晉陽之戰) was fought between the elite families of the State of Jin, the house of Zhao and the house of Zhi (智), in the Spring and Autumn period of China. The other houses of Wei and Han first participated in the battle in alliance with the Zhi, but later defected to ally with Zhao to annihilate the Zhi house. This event was a catalyst to the Tripartition of Jin (三家分晉) in 434 BC, the forming of the three states of Zhao, Wei, and Han, and the start to the Warring States period. It is the first battle described in the Song Dynasty history compendium Zizhi Tongjian
By 490 BC, after the destruction of the houses of Fan (范) and Zhonghang (中行), control of the State of Jin, then the largest state in China, was contested by four elite families: Zhi, Wei, Zhao, and Han. With multiple military victories under his belt, Zhi Bo Yao (智伯瑤) of the house of Zhi exerted the most influence in the Jin court — all decisions of the state had to pass through him. He also controlled the most territory within the state. The reigning duke of Jin, Duke Ai, was powerless to restrain him. So Zhi Bo, in his pride, began to demand lands from the other three houses. The houses of Wei and Han reluctantly complied to evade Zhi Bo's wrath, but Viscount Xiang of Zhao (趙襄子) refused to cede the territories of Lin (藺) and Gaolang (皋狼), both in modern-day Lishi, to Zhi. Zhi Bo, in retribution, formed a secret alliance with the houses of Wei and Han to attack Zhao.
Viscount Xiang suspected an attack from Zhi Bo, since he has heard that Zhi Bo sent envoys to Han and Wei three times, but never to Zhao. After rejecting suggestions to move to Zhangzi (長子, in modern-day Changzhi) or Handan, the viscount asked his minister Zhang Mengtan (張孟談) where he could prepare his defence, and Zhang Mengtan suggested Jinyang because Jinyang had been well-governed for generations. Viscount Xiang agreed, and summoned Yanling Sheng (延陵生) to lead the army carriages and cavalry ahead to Jinyang, the viscount himself to follow later. Once in Jinyang, the viscount, following the suggestions of Zhang Mengtan, issued orders to refill the granaries and the treasuries, repair walls, make arrows, and melt copper pillar for metal. By virtue of past governance, the treasuries, granaries, and arsenals were filled within three days, and the walls repaired within five. Thus all of Jinyang was prepared for war
When the three armies of Zhi, Wei, and Han reached Jinyang in 455 BC, they laid siege to the city, but for three months they could not take the city. They then fanned out and surrounded the city, and a year later diverted the flow of the Fen River to inundate the city. All buildings under three storeys high were submerged, and the people of Jinyang were obliged to live in nest-like perches above the water and hang their kettles from scaffoldings in order to cook.
By the third year, supplies had run out for the Zhao, diseases broke out, and the populace were reduced to eating each other's children. Although the common people remained firm in the defence, the court ministers' loyalties began to waver. Viscount Xiang asked Zhang Mengtan, "Our provisions are gone, our strength and resources are exhausted, the officials are starving and ill, and I fear we can hold out no longer. I am going to surrender the city, but to which of the three states should I surrender?" Zhang Mengtan, much alarmed, persuaded Viscount Xiang not to surrender but instead send him out to negotiate with the houses of Wei and Han.
The houses of Wei and Han were promised an even split of Zhao's territories when the battle was won, but both the Wei and Han leaders were uneasy, since they understood that they would be soon to follow if Zhao fell to Zhi. Zhi Bo's minister, Yu Chi (郤疵), warned Zhi Bo that the two houses were going to revolt, since "the men and horses [of Jinyang] are eating each other and the city is soon to fall, yet the lords of Han and Wei show no signs of joy but instead are worried. If those are not rebellious signs, then what are they?" Zhi Bo paid Yu Chi no heed, and instead told the lords of Han and Wei of Yu Chi's suspicion. Yu Chi, knowing that his warning fell to deaf ears, excused himself from the battlefield by going to the State of Qi as an envoy.
Indeed, when Zhang Mengtan secretly met with Viscount Huan of Wei (魏桓子) and Viscount Kang of Han (韓康子), the viscounts confessed that they were secretly planning to mutiny against Zhi. The three discussed their plans and settled on a date to execute the plans. Zhang Mengtan returned to Jinyang to report back to Viscount Xiang, and the viscount, in joy and apprehension, bowed to Zhang several times as a sign of great reverence.
One of Zhi Bo's clansmen, Zhi Guo (智過), chanced to see the viscounts of Wei and Han after the secret meeting, and warned Zhi Bo of the possibility that they might rebel, judging by their lack of restraints like before. Zhi Bo again chose to put his trust in the two viscounts, saying: "Since I have been this good to them, they would surely not attack or deceive me. Our troops have invested Jinyang for three years. Now when the city is ready to fall at any moment and we are about to enjoy the spoils, what reason would they have for changing their minds?" Zhi Bo told the two viscounts what Zhi Guo said, and the two viscount learnt to be cautious when they saw Zhi Guo the next day. Zhi Guo, seeing the change in their looks, insisted to Zhi Bo that the two ought to be executed. Zhi Bo would not hear of it, and Zhi Guo suggested another plan to buy their friendship: to bribe the influential ministers Zhao Jia (趙葭) of Wei and Duan Gui (段規) of Han with enfeoffement of the Zhao lands. Zhi Bo rejected the proposal because the Zhao lands were going to split in three already, and he did not want to receive less than one third of the eventual spoils. Seeing that Zhi Bo would not listen, Zhi Guo left Zhi Bo and changed his surname to Fu (輔) as a precaution.
Hearing this, Zhang Mengtan urged Viscount Xiang to take action immediately, lest Zhi Bo changes his mind. The viscount then dispatched Zhang Mengtan to the camps of Wei and Han, alerting them of the time of the final attack. On the night of May 8th, 453 BC, Zhao troops killed the men guarding the dams of the Fen River and let the river flood the Zhi armies. As the Zhi armies fell into chaos trying to stop the water, the Wei and Han armies attacked Zhi from the sides and the Viscount of Zhao led his soldiers in a frontal attack. Together they inflicted a severe defeat on Zhi Bo's army and took him prisoner.
Viscount Xiang had a grudge on Zhi Bo because Zhi Bo had often humiliated him in the past, thus Viscount Xiang executed Zhi Bo and made his skull into a winecup. No one in the house of Zhi was spared save for Zhi Guo's family, who had already changed their surnames and fled. The territories of Zhi were evenly distributed among the three victors
With the elimination of the Zhi house, control of the State of Jin fell to the remaining three families, their powers unchecked by anyone in the state. In 434 BC, following the death of Duke Ai, the three families annexed all of Jin's lands, leaving only the capital estates of Jiang (絳) and Quwo (曲沃), both in today's Yuncheng, for the next duke of Jin. In 403 BC, the Wei, Zhao and Han lords all went to King Weilie of Zhou in Luoyang and were made marquises in their own right, establishing the three states of Zhao, Wei, and Han, ushering in the beginning of the Warring States period by Sima Guang's definition. Most historians, when referring to those three states, call them the "Three Jins" (三晉). The State of Jin continued to exist with a tiny piece of territory until 376 BC when the rest of the territory was partitioned by the Three Jins.
The Legalist thinker Han Feizi of the late Warring States period used this battle as an example of failure via greed and perversity, one of the "Ten Faults" that a ruler should not have. He reasoned that because Zhi Bo was too fond for profit, he opened himself to the destruction of the state and his own demise.
The Song Dynasty statesman Sima Guang, in his Zizhi Tongjian, attribute Zhi Bo's failure to his lacking virtue compared to his talents, and thus invited disaster