哀兵必胜 A sad army will always win
Explanation
指处于悲愤、正义、绝望等情绪中的军队，由于其士气高涨，战斗力强，往往能战胜敌人。
Refers to an army in a state of grief, justice, despair, etc., often able to defeat the enemy because of their high morale and combat effectiveness.
Origin Story
战国时期，燕国与齐国交战，燕国屡战屡败，士气低落。太子丹派荆轲刺秦王，虽然刺杀失败，但此举却激起了燕国人民的悲愤之情，他们团结一心，决心与齐国决一死战。最终，凭借着这股“哀兵”之势，燕国反败为胜，重创齐军。这段历史也成为了“哀兵必胜”的典型例证，说明了在正义和悲愤的驱使下，即使是处于劣势的军队，也能爆发出惊人的力量。
During the Warring States period, the kingdom of Yan and Qi were at war, and Yan repeatedly lost battles, morale was low. Prince Dan sent Jing Ke to assassinate King Qin, although the assassination failed, but this action aroused the anger and grief of the people of Yan, they united and decided to fight Qi to the death. In the end, with this "sad soldier" momentum, Yan reversed its defeat into victory, and severely damaged the Qi army. This period of history has also become a typical example of "a sad army will always win", showing that even an inferior army, driven by justice and anger, can erupt with amazing power.
Usage
常用于形容军队，也可以引申到其他领域，比喻处于劣势但充满正义感和悲愤情绪的一方最终会取得胜利。
Often used to describe armies, but can also be extended to other areas, to describe the party that is at a disadvantage but is full of a sense of justice and indignant emotions and eventually wins.
Examples
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面对强敌，他们士气高昂，仿佛一支哀兵，最终取得了胜利。
miàn duì qiáng dí, tāmen shì qì gāo áng, fǎng fú yī zhī āi bīng, zuì zhōng qǔ dé le shèng lì.
Faced with a strong enemy, their morale was high, like an army of sorrow, and they finally won the victory.
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历史上，许多看似弱小的军队，凭借着悲壮的气势，取得了以弱胜强的战役。
lìshǐ shàng, xǔ duō kànshì ruò xiǎo de jūnduì, píngjiè zhe bēi zhuàng de qìshì, qǔdé le yǐ ruò shèng qiáng de zhànyì
In history, many seemingly weak armies have achieved victories against stronger opponents through their tragic momentum..