During the Warring States period, wars were continuous, and various feudal states adopted strategies of "vertical and horizontal alliances" to strengthen or preserve their power. In this context, a group of outstanding strategists emerged, and Zhang Yi was one of them.
Zhang Yi was a native of the State of Wei, and his ancestors were Wei nobility. He studied the art of vertical and horizontal alliances alongside Su Qin under the tutelage of Guiguzi. Well-read and brimming with strategic insight, even the eloquent Su Qin admitted he was no match for Zhang Yi. After completing their studies, Su Qin introduced his strategy of vertical alliances and quickly gained fame among the various states, while Zhang Yi remained an obscure retainer in the household of the Prime Minister of Chu. However, half a year later, Zhang Yi's reputation rivaled Su Qin's, and he earned the favor of the King of Qin, securing a high-ranking position. His rapid rise to fame was largely due to a significant incident that happened to him.
Once, after the Prime Minister of Chu returned victorious from battle, the King of Chu rewarded him with a jade bi. That day, while feasting and drinking with his retainers, the Prime Minister gleefully showed the jade bi to his guests. Before the banquet ended, however, the jade bi disappeared. Someone then suggested to the Prime Minister that Zhang Yi, being poor, must have stolen it. The Prime Minister agreed and had Zhang Yi arrested and interrogated. Zhang Yi had not stolen the Heshi bi, so no matter how brutally the Prime Minister tortured him, he refused to confess. Finally, beaten black and blue, Zhang Yi was released when the Prime Minister feared he might die. Upon returning home, Zhang Yi asked his wife, "Is my tongue still here?" His wife replied, "Yes." Zhang Yi said, "As long as my tongue is intact, I will have my day."
In 329 BCE, Zhang Yi arrived in the State of Qin and was appointed as a guest minister by King Huiwen of Qin. That year, Qin attacked the State of Wei and captured Puyang. Zhang Yi proposed his policy of "vertical alliance" and advised the Qin king to win over Wei. He then suggested returning Puyang to Wei and sending Prince Yao to Wei as a hostage. After arriving in Wei, Zhang Yi said to the Wei king, "Your Majesty, you can see that the Qin king sincerely seeks good relations with Wei! He not only returns a city but also sends his son as a hostage. As a matter of courtesy, how will Wei repay Qin?" The Wei king asked how to repay. Zhang Yi replied, "Our king only loves land. If you gift some territory to Qin, the Qin king will surely regard you as a brother. Then, when Qin and Wei attack other states together, the land you gain after victory will far exceed what you give to Qin!" These words delighted the Wei king, who immediately ceded fifteen counties to Qin, along with a key military stronghold.
When Zhang Yi returned to the State of Qin, he was immediately promoted to Prime Minister. Soon after, the King of Qin ordered Zhang Yi to lead an army to attack the State of Wei, causing great panic for the King of Wei. The following year, the King of Wei planned to ally with Qi to resist Qin. Shortly thereafter, Wei formed an alliance with Han, Zhao, Yan, and Zhongshan—five states in total—but under Zhang Yi's instigation, Chu launched an attack on Wei, breaking up the alliance. Wei sought help from Qi, but Zhang Yi again sowed discord, resulting in Qi not only refusing to aid Wei but also joining Chu in attacking it. In 323 BC, Zhang Yi won over the King of Wei, who once again agreed to befriend Qin. The next year, to further subjugate Wei to Qin, Zhang Yi resigned as Qin's Prime Minister and went to Wei. The King of Wei, believing in Zhang Yi's abilities, immediately appointed him as Prime Minister of Wei. Over four years, Zhang Yi finally persuaded the King of Wei to submit to Qin. After achieving his goal, Zhang Yi returned to Qin, where the King of Qin continued to appoint him as Prime Minister.
At this time, the State of Qi had defeated the states of Zhao and Wei, becoming a powerful force in the east, and soon formed an alliance with the State of Chu. This Qi-Chu alliance became a thorn in the side of the King of Qin. Thus, Zhang Yi was once again the perfect candidate to break apart the Qi-Chu alliance. In 313 BC, Zhang Yi resigned from his position as Prime Minister of Qin and went to Chu. He first befriended King Huai of Chu's favored ministers, using them to gain the king's trust, and eventually approached King Huai himself. Zhang Yi said to King Huai: "Our King of Qin wishes to be on good terms with Your Majesty, but the person he hates most is the King of Qi. Now that Chu and Qi have formed an alliance, our King of Qin cannot befriend you. If Your Majesty breaks relations with Qi, then I will ask the King of Qin to cede a territory of six hundred square li to Chu. Then Chu and Qin can join forces to attack Qi, allowing Chu to seize some of Qi's lands. Moreover, Your Majesty will earn the favor of Qin. This is a triple win—why not do it?" King Huai of Chu, delighted by Zhang Yi's words, immediately agreed. Although some ministers advised King Huai not to be deceived by Zhang Yi's smooth talk, the king refused to listen and insisted on appointing Zhang Yi as his Prime Minister. Subsequently, King Huai announced the severance of relations with Qi and sent envoys to accompany Zhang Yi back to Qin to receive the promised land.
After spending three months in the state of Qin, Zhang Yi consistently claimed illness and refused to attend court. King Huai of Chu suspected that Zhang Yi was dissatisfied because Chu had not completely severed ties with Qi, so he specially sent someone to Qi to insult the King of Qi. The King of Qi was furious and declared that he would never maintain friendly relations with Chu. Subsequently, the King of Qi also sent an envoy to Qin, proposing an alliance to attack Chu. At this point, King Huai of Chu again sent someone to demand the 600 li of land from Zhang Yi. Seeing that his goal had been achieved, Zhang Yi reneged, saying, "I said six li, not six hundred li; the King of Chu misheard." The envoy returned and reported this to King Huai of Chu, who flew into a rage and immediately prepared to launch an attack on Qin. However, the Qi-Chu alliance had already been broken, and Chu could no longer count on Qi's support. In 312 BC, a major war broke out between Qin and Chu, and the Chu army suffered a crushing defeat. In his fury, King Huai of Chu mobilized his entire army to attack Qin again, only to be defeated once more. At this time, the states of Han and Wei also took the opportunity to attack Chu. Surrounded by enemies on all sides, Chu was ultimately forced to cede territory to Qin and sue for peace.
At that time, among the six states, Wei, Qi, and Chu had already submitted to the Qin state, leaving only Han, Zhao, and Yan. Subsequently, Zhang Yi visited these remaining states one by one, and soon their power was also weakened. Although the six states later formed a brief alliance, due to Zhang Yi's earlier sowing of discord, they were already united in name only. Thus, under the relentless attacks of the powerful Qin state, the six states ultimately suffered a crushing defeat.
Later, King Huiwen of Qin passed away, and the crown prince ascended the throne, becoming King Wu of Qin. When King Wu was still a prince, he disliked Zhang Yi, and after he became king, Zhang Yi fled to the State of Wei, where the King of Wei appointed him as Prime Minister. The following year, Zhang Yi died of illness. With his exceptional eloquence and wisdom, Zhang Yi dismantled Su Qin's strategy of vertical alliance, making an indelible contribution to Qin's unification of the realm.