乘虚而入 To take advantage of the situation and enter
Explanation
指趁着对方空虚、疏漏的时候，迅速地进去或采取行动。多用于军事和政治方面，也可用在日常生活中。
To take advantage of the opponent's weakness and gaps to quickly enter or take action. It is often used in military and political contexts, but it can also be used in daily life.
Origin Story
话说唐朝时期，淮西节度使吴元济造反，朝廷派大将李愬前去讨伐。吴元济凭借地利，倚仗坚城固垒，自恃难攻，多次击败唐军。李愬深知强攻难以奏效，便改用策略。他先派人四处散布流言，说唐军将要从其他方向进攻，迷惑吴元济。同时，他暗中收买吴元济身边的官员和士兵，瓦解其内部势力。趁着吴元济放松警惕，李愬率领精兵，于深夜奇袭蔡州，直捣黄龙，将吴元济擒获。这场战争，李愬正是抓住了吴元济的疏忽大意，乘虚而入，取得了最终的胜利。
During the Tang Dynasty, Wu Yuanji, the military governor of Huai-Xi, rebelled, and the court sent the great general Li Su to quell the rebellion. Wu Yuanji, relying on his geographical advantages and strong fortifications, considered himself invincible and repeatedly defeated the Tang army. Li Su knew that a direct attack would be ineffective, so he changed his strategy. He first sent people to spread rumors that the Tang army would attack from other directions, confusing Wu Yuanji. At the same time, he secretly bought over Wu Yuanji's officials and soldiers, undermining his internal forces. Taking advantage of Wu Yuanji's complacency, Li Su led his elite troops in a surprise night attack on Caizhou, directly attacking the enemy's headquarters and capturing Wu Yuanji. In this war, Li Su seized Wu Yuanji's negligence and complacency, taking advantage of the situation to achieve victory.
Usage
常用于军事或政治斗争的描写，也可用于形容个人在生活或工作中抓住机会。
Often used in the description of military or political struggles, it can also be used to describe how individuals seize opportunities in life or work.
Examples
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敌军乘虚而入，我军猝不及防，损失惨重。
díjūn chéngxū'ér rù, wǒjūn cùbùjífáng, sǔnshī cǎnzhòng
The enemy took advantage of the situation and broke in, our army was caught off guard and suffered heavy losses.
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他乘虚而入，成功地说服了对方。
tā chéngxū'ér rù, chénggōng de shuōfú le duìfāng
He took advantage of the situation and successfully persuaded the other party.
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公司内部出现矛盾，竞争对手乘虚而入，抢占了市场份额。
gōngsī nèibù chūxiàn máodùn, jìngzhēng duìshǒu chéngxū'ér rù, qiǎngzhàn le shìchǎng fèn'é
Conflicts within the company allowed competitors to seize market share.