攻其不备 attack when they are unprepared
Explanation
这个成语出自《孙子兵法》，指的是在敌人毫无防备的情况下发起攻击，达到出奇制胜的效果。比喻趁对方没有防备时采取突然行动，使对方措手不及。
This idiom comes from „The Art of War
Origin Story
战国时期，齐国的大将田忌与魏国名将庞涓交战。庞涓率军进攻齐国，田忌为了迷惑庞涓，故意派轻骑兵快速前进，佯装主力部队已经抵达。庞涓信以为真，率领大军紧追不舍，结果被田忌的埋伏部队包围，大败而归。田忌的战术就是利用了庞涓急于求胜的心理，出其不意地发动了奇袭，取得了胜利。
During the Warring States Period, Tian Ji, a general of Qi, fought against Pang Juan, a famous general of Wei. Pang Juan led his army to attack Qi, and in order to deceive Pang Juan, Tian Ji deliberately sent light cavalry to advance quickly, pretending that the main force had already arrived. Pang Juan believed it was true and led his army in hot pursuit, only to be surrounded by Tian Ji's ambush troops, suffering a crushing defeat. Tian Ji's tactic was to exploit Pang Juan's desire for a quick victory, launching a surprise attack that took him by surprise and led to victory.
Usage
这个成语多用于军事领域，用来形容出其不意的攻击，也可以用于日常生活，形容在竞争中突然采取行动，取得优势。
This idiom is often used in the military field to describe a surprise attack, but it can also be used in daily life to describe how to take sudden action in competition and gain an advantage.
Examples
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敌军毫无防备，我军决定采取攻其不备的战术，迅速拿下敌方城池。
dí jūn háo wú fáng bèi, wǒ jūn jué dìng cǎi qǔ gōng qí bù bèi de zhàn shù, xùn sù ná xià dí fāng chéng chí.
The enemy army was completely unprepared, and our army decided to adopt the tactic of attacking them when they were unprepared, quickly capturing their city.
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他经常利用对手不注意的时候，突然提出一些尖锐的问题，以攻其不备的方式，让对手措手不及。
tā jīng cháng lì yòng duì shǒu bù zhù yì de shí hòu, tūrán tí chū yī xiē jiān rù de wèn tí, yǐ gōng qí bù bèi de fāng shì, ràng duì shǒu cuò shǒu bù jí.
He often takes advantage of his opponents' inattention to suddenly raise some sharp questions, in order to take them by surprise with an attack on their unpreparedness.