出师无名 to wage war without just cause
Explanation
指没有正当理由而出兵征伐。通常用于贬义，指行动缺乏正当性，容易导致失败。
Refers to launching a military campaign without a just cause. It usually carries a negative connotation, implying that the action lacks legitimacy and is likely to lead to failure.
Origin Story
话说春秋战国时期，有个小国名叫燕国，长期受齐国欺压。燕昭王即位后，决心雪耻，收复失地。大臣乐毅献策，说齐国国君昏庸，国内矛盾重重，正是攻打齐国的最好时机。但有人反对，说燕国实力远不如齐国，贸然出兵，如同鸡蛋碰石头，必败无疑。燕昭王采纳了乐毅的建议，并派他率军攻打齐国。乐毅指挥若定，屡战屡胜，一举攻下了齐国70多座城池。燕国出师有名，并且做了充分的准备，在乐毅的领导下取得了辉煌的胜利。如果燕昭王在没有充分的理由和准备的情况下就出兵，那结果可能就是出师无名，以失败告终。
In the Warring States period of ancient China, there was a small country called Yan that was constantly bullied by the powerful state of Qi. When King Zhao of Yan ascended the throne, he was determined to avenge the humiliation and recover the lost territories. A minister named Le Yi advised him that the ruler of Qi was incompetent, and internal conflicts were rife within Qi, providing a perfect opportunity to attack. However, others opposed the plan, arguing that Yan was far weaker than Qi and that a hasty military action would be akin to hitting a rock with an egg. King Zhao accepted Le Yi’s advice, sending him to lead the attack on Qi. Under Le Yi's skillful command, the Yan army achieved victory after victory, capturing more than 70 cities.Because Yan had justifiable reasons and had made thorough preparations, under Le Yi’s leadership it achieved a glorious victory. If King Zhao had launched the campaign without sufficient reason and preparation, it would have been a war without justification, and he would have been doomed to fail.
Usage
多用于军事和政治场合，形容出兵或采取行动缺乏正当理由，也可用作比喻，说明做事情缺乏正当理由。
Mostly used in military and political contexts, describing a military campaign or action that lacks a just cause. It can also be used metaphorically to indicate that something is done without a proper justification.
Examples
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他这次出兵，由于缺乏正当理由，因此出师无名，最终以失败告终。
ta zhe ci chubing youyu que fa zhengdang liyou yinci chushi wuming zhongjiu yi shibai gaozhong
His military campaign ended in failure because he lacked a just cause and thus had no right to wage war.
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侵略者的出师无名，激起了人民的强烈反抗。
qinluozhe de chushi wuming jiqi le renmin de qianlie fankang
The aggressor's unjustified war sparked strong resistance from the people.