During the Warring States period, the Eastern Zhou dynasty grew increasingly weak. As a result, feudal lords began vying for supremacy, with the state of Qin being the most powerful. In this context, two schools of political strategists emerged: the "Vertical Alliance" advocates, who urged weaker states to unite against Qin, and the "Horizontal Alliance" advocates, who urged weaker states to follow Qin in conquering other weak states.
Zhang Yi, the prime minister of the Qin state, realized that the alliance of the six states—Qi, Chu, Yan, Zhao, Han, and Wei—was unshakable. He resigned from his post and set out for Wei, determined to persuade its king to abandon the coalition and forge a bond with Qin instead.
Zhang Yi arrived in the state of Wei and was appointed prime minister the following year. Though serving in Wei, his heart remained with Qin, as he constantly sought to make Qin dominate the world through the "Horizontal Alliance" strategy. He urged the King of Wei to ally with Qin and attack states like Qi and Chu. However, the King of Wei, aware of Qin's great ambition and lack of trustworthiness, did not follow his advice.
Enraged by the news, the King of Qin dispatched a massive army to attack Wei while secretly sending a steady stream of treasures to Zhang Yi. Zhang Yi accepted the gifts but felt deeply ashamed, having no way to repay them. After King Xiang of Wei died, Zhang Yi urged the new King Ai of Wei to serve Qin, but his advice was again rejected.
Zhang Yi secretly urged the state of Qin to attack Wei. After Wei was defeated, they were invaded by Qi the following year and lost again. Seizing the opportunity, Qin launched another assault on Wei, first crushing the army of the Han general Shen Cha, killing 82,000 men, which sent shockwaves through all the feudal lords.
Zhang Yi, seizing the opportunity presented by Wei's repeated defeats and the weakening of the allied states, combined with Qin's military pressure, advised King Ai of Wei. He analyzed Wei's geography, military strength, precarious situation, and the unreliability of the "Vertical Alliance," using the metaphor "accumulated feathers can sink a ship" to warn that if Wei ignored these disadvantages, it would face the same danger as a vessel overwhelmed by a buildup of light feathers.
King Ai of Wei, attacked from both sides, had no choice but to abandon the "Vertical Alliance" agreement and request a "Horizontal Alliance" with the State of Qin. Later, people used the idiom "Feathers Sink a Boat" to describe how small bad things, when accumulated, can lead to serious consequences.
Source: *Strategies of the Warring States*, Chapter "Strategies of Wei I"
Meaning of the Idiom: Later, the Chinese idiom "积羽沉舟" came to describe how small bad things, when accumulated, can lead to serious consequences.