大敌当前 a formidable enemy is at hand
Explanation
意思是强敌就在眼前。形容情况危急，形势严峻。
It means a strong enemy is right in front of you. It describes a critical situation and a serious situation.
Origin Story
话说三国时期，蜀汉丞相诸葛亮率军北伐，与魏国大军在五丈原对峙。蜀军兵力不足，粮草供应困难，而魏军兵强马壮，诸葛亮深知大敌当前，形势危急。他一方面励精图治，稳定军心，另一方面积极寻求战略突破口，最终凭借智慧和谋略，将魏军击退。然而，这场战争也耗尽了蜀汉的国力，诸葛亮最终病逝于军中，留下千古遗憾。
During the Three Kingdoms period, Zhuge Liang, the chancellor of Shu Han, led his troops on a northern campaign and confronted the large Wei army at Wuzhangyuan. The Shu army was understaffed, with difficulties in supplying food and fodder, while the Wei army was strong and well-equipped. Zhuge Liang was well aware that a major enemy was at hand and the situation was critical. He worked hard to improve governance, stabilise army morale, and actively sought strategic breakthroughs. Eventually, through wisdom and strategy, he repelled the Wei army. However, the war had exhausted Shu Han's national strength, and Zhuge Liang eventually died in the army, leaving behind a thousand-year-old regret.
Usage
作定语、状语；形容情况危急，形势严峻。
As a predicate or adverbial modifier; to describe a dangerous situation and a serious situation.
Examples
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大敌当前，我们必须团结一致，共御外侮。
dà dí dāng qián, wǒmen bìxū tuánjié yīzhì, gòng yù wàiwǔ
In the face of a formidable enemy, we must unite and defend against foreign aggression.
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面对大敌当前的险峻形势，他临危不惧，指挥若定。
miàn duì dà dí dāng qián de xiǎnjùn xíngshì, tā língwéi bùjù, zhǐhuī ruòdìng
Facing the dangerous situation with a major enemy at hand, he remained calm and composed, maintaining command.