During the late Spring and Autumn period, the states of Wu and Yue were locked in constant warfare. King Fuchai of Wu raised an army to attack Yue, and King Goujian of Yue, knowing his forces were no match for Wu's, heeded his minister Wen Zhong's strategy. Goujian sent Zhu Jiyin to sue for peace with Wu as a delaying tactic.
King Fuchai of Wu, moved by the King of Yue's tactful and humble plea for peace, told his ministers, "I am about to launch a major campaign against Qi, so I intend to accept Yue's peace offer."
Wu Zixu warned, 'Your Majesty, you must not agree to peace—Yue is not sincere. Their minister Wen Zhong knows you thrive on strength and victory, so he speaks with deliberate humility to flatter your pride, making you overconfident in your quest to dominate the central states. This will drain Wu's power while they wait to seize our kingdom. King Goujian of Yue is a trustworthy ruler who loves his people; everyone supports him, the harvests are good, and his power grows daily. We must strike while we still have the strength to defeat them. If we don't kill the snake while it's small, how will we deal with it once it becomes a serpent?'
King Wu disagreed, saying, "Why do you value Yue so highly? Is Yue truly worth such worry? Without Yue, to whom would I show off my military might?" So King Wu agreed to make peace with Yue.
As Wu Zixu had predicted, the state of Yue steadily strengthened itself, exploiting Wu's campaign against Qi to drain its resources, and ultimately defeated Wu to claim hegemony.
The idiom "If you don't kill a viper, what will you do when it becomes a serpent" means if a small snake is not killed, it becomes uncontrollable when grown. It warns that if an enemy is not eliminated while weak, it will cause future trouble.
Source: *Guoyu*, "Discourses of Wu"
Meaning of the Idiom: Later, the Chinese idiom "为虺弗摧，为蛇若何" came to describe if a small snake is not killed, it becomes uncontrollable when grown.