兵败如山倒 A defeat like a collapsing mountain
Explanation
形容军队溃败，像山一样倒塌，一败涂地。比喻失败极其彻底，不可挽回。
Describes the complete collapse of an army, as if a mountain were collapsing. It's used to describe a thorough and irreversible failure.
Origin Story
话说三国时期，蜀汉丞相诸葛亮率领大军北伐曹魏，意图一举收复中原。然而，魏军将领司马懿早已料到蜀军此举，设下埋伏，以逸待劳。蜀军深入敌境，遭遇魏军猛烈反击，一时间蜀军阵脚大乱，兵败如山倒，无数蜀军将士倒在了血泊之中。诸葛亮无奈之下，只得下令撤退，北伐失败。这次失败，对蜀汉来说，打击巨大，元气大伤，从此再无力与魏军抗衡，最终走向灭亡。诸葛亮的这次北伐，正应了那句兵败如山倒，惨痛的教训令人扼腕叹息。
During the Three Kingdoms period, Zhuge Liang, the chancellor of Shu Han, led a large army to attack Cao Wei, intending to recapture the Central Plains. However, Sima Yi, a general of Wei, had already anticipated this move, setting an ambush and waiting for the exhausted army. The Shu army went deep into enemy territory, and suffered a fierce counterattack from the Wei army. For a time, the Shu army's formation was thrown into chaos, suffering a disastrous defeat with countless soldiers falling in a pool of blood. Zhuge Liang had no choice but to order a retreat, the northern expedition ending in failure. This defeat was a heavy blow to Shu Han, causing great damage and leaving it unable to fight against the Wei army, eventually leading to its downfall. Zhuge Liang's northern expedition was a perfect example of the idiom, "兵败如山倒", the painful lesson is enough to make people sigh with regret.
Usage
多用于描写军队溃败的场景，也可用作比喻，形容失败的彻底。
Mostly used to describe the collapse of an army, but can also be used metaphorically to describe a thorough failure.
Examples
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面对敌人的猛烈进攻，我军兵败如山倒，损失惨重。
miànduì dí rén de měngliè jìngōng, wǒ jūn bīng bài rú shān dǎo, sǔnshī cǎnzhòng.
Faced with the enemy's fierce attack, our army suffered a disastrous defeat, with heavy losses.
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这次比赛，我们兵败如山倒，输得一塌糊涂。
zhè cì bǐsài, wǒmen bīng bài rú shān dǎo, shū de yītātútu
In this competition, we were defeated and lost badly