里应外合 inside and outside cooperation
Explanation
"里应外合"是一个成语，意思是内部人员接应，外部人员配合进攻，共同取得胜利。它形容内外相互配合，共同攻打，多指军事行动。
"Li Ying Wai He" is an idiom that means internal personnel respond and external personnel cooperate to attack, jointly achieving victory. It describes the mutual cooperation between the inside and outside, jointly attacking, mostly referring to military actions.
Origin Story
话说三国时期，蜀国大将诸葛亮北伐，兵临城下，却久攻不下。原来，城内有魏军内应，与诸葛亮军里应外合。魏军内应在城内打开城门，诸葛亮大军长驱直入，魏军溃不成军，最终取得了胜利。这便是历史上有名的"里应外合"战例。
During the Three Kingdoms period, Zhuge Liang, a Shu general, launched a northern expedition and besieged a city, but he could not capture it for a long time. It turned out that there was a Wei general inside the city who cooperated with Zhuge Liang’s army. This Wei general opened the city gate, and Zhuge Liang's army stormed into the city,defeating the Wei army and ultimately winning the battle. This is a famous example of "Li Ying Wai He" in history.
Usage
"里应外合"常用于形容军事行动中内部配合外部的战术，也用于比喻内部外部相互配合，共同完成某项任务。
"Li Ying Wai He" is often used to describe the tactic of internal cooperation with external forces in military operations, but also figuratively to describe the mutual cooperation between the inside and outside to complete a task together.
Examples
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这次行动，内外勾结，里应外合，才能成功。
zhe ci xingdong nei wai goujie li ying wai he cai neng chenggong
This operation succeeded because of inside and outside collusion.
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敌军采用里应外合的战术，很快攻破了城池。
di jun caiyong li ying wai he de zhanshu hen kuai gongpo le chengchi
The enemy forces used the tactic of inside and outside cooperation and quickly captured the city.