During the Warring States period, the state of Qin, after implementing Shang Yang's reforms, rapidly grew powerful and leaped to become the strongest state of all.
The state of Qin grew powerful, and its ambition to dominate the world swelled day by day, posing a threat to the six states east of the Xiaoshan Mountains. Driven by common interests, the six states formed a united front stretching from Yan in the north to Chu in the south, adopting the strategy of "uniting the weak to attack the strong."
In theory, this strategy was sound: the six warring states together possessed five times the land and ten times the troops of Qin, so if they maintained their alliance, they could effectively stop Qin from swallowing the entire realm.
The political strategist Su Qin tirelessly traveled between the warring states, using his eloquence to forge alliances. He ultimately succeeded and was honored as prime minister by all six kingdoms.
When Su Qin arrived in the state of Chu, he spoke at length to King Wei of Chu, analyzing the pros and cons of joining the Vertical Alliance. He declared, "Chu is the greatest power under heaven, and you, Your Majesty, are the wisest ruler. What Qin fears most is Chu. If Chu is strong, Qin must be weak; if Chu is weak, Qin will grow strong—the two cannot coexist, so Qin sees Chu as its most formidable rival."
King Wei of Chu pondered Su Qin's reasoning. Su Qin continued, "I believe it would be better for Your Majesty to join the Vertical Alliance, isolating Qin. With the alliance, Chu remains a great power. Otherwise, Qin will use the Horizontal Alliance to divide and conquer, fulfilling its hegemonic ambitions, and by then, Chu will not be able to hold on."
Su Qin’s words struck a chord with King Wei of Chu. Deeply alarmed by Qin’s expansionist ambitions—especially since Chu shared a border with Qin, which had long coveted the lands of Ba and Shu and sought to swallow up Hanzhong—the king realized that Chu was no match for Qin in military strength and could never defeat it by force. Facing an irreconcilable rivalry, the only way for Chu to stand on its own and grow strong was to unite with the other feudal states against Qin. Thus, King Wei of Chu agreed to Su Qin’s proposal and joined the Vertical Alliance.
In reality, the six states were rife with internal conflicts, each hoping to gain an advantage at the expense of the others, so the alliance was extremely fragile. Su Qin's painstakingly crafted "Vertical Alliance" lasted only three years before collapsing. Later, Chu, deceived by Zhang Yi, angrily attacked Qin and suffered a crushing defeat. King Huai of Chu was then detained in Qin, and from that point, Chu weakened beyond recovery, perfectly validating Su Qin's assertion: "If Chu weakens, Qin strengthens—the two cannot coexist." The phrase "irreconcilable opposition" describes a situation where two hostile parties are so sharply opposed that neither can tolerate the other's existence, also used to signify an irreconcilable conflict.
Source: *Strategies of the Warring States*, Chapter "Strategies of Chu"
Meaning of the Idiom: Later, the Chinese idiom "势不两立" came to describe how two hostile parties are so sharply opposed that neither can tolerate the other's existence, also used to signify an irreconcilable conflict.