功败垂成 Failure at the verge of success
Explanation
事情到了快要成功的时候却失败了。形容事情接近成功的时候，由于某种原因而失败，令人惋惜。
Things failed when they were about to succeed.It describes a situation where something was almost accomplished but failed due to some reason, which is regrettable.
Origin Story
话说东晋名将谢安，在淝水之战中以少胜多，大败前秦苻坚，眼看就要统一北方，却因朝廷猜忌，被召回建康，功亏垂成，抱憾终身。这个故事告诉我们，做事要谨慎，也要看清形势，不要因小失大。谢安指挥若定，运筹帷幄，本可以彻底击溃苻坚，统一北方，然而东晋朝廷的猜忌和内乱，最终导致了他的功败垂成。历史的悲剧也给后人以警示，切不可轻敌大意，更不可因个人私欲而误国误民。
Xie An, a famous general of the Eastern Jin Dynasty, won a great victory in the Battle of Fei River, defeating the former Qin army led by Fu Jian. He was on the verge of unifying the north, but due to court intrigue, he was recalled to Jiankang, resulting in his unfinished ambition. This story teaches us the importance of prudence, understanding the bigger picture and avoiding self-defeating actions.Xie An's masterful strategic planning and calm demeanor could have led to the complete defeat of Fu Jian and the unification of the north. But the jealousy and internal conflicts within the court of the Eastern Jin Dynasty ultimately led to his failure to achieve his goals. This historical tragedy serves as a warning against underestimating one's opponent, and against letting personal ambitions harm the country.
Usage
常用来形容事情在即将成功的时候失败了，含有惋惜之意。多用于叙事或议论。
It is often used to describe that something failed at the moment of success, which is regrettable. Mostly used in narratives or discussions.
Examples
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他多年的努力，眼看就要成功了，谁知功败垂成，真是令人惋惜！
ta duonian de nuli, yan kan jiu yao chenggong le, shui zhi gong bai chui cheng, zhen shi ling ren wanxi!
After years of effort, it was about to succeed, but unfortunately, it ended in failure, which is a pity!
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经过几年的努力，眼看就要完成目标了，没想到功败垂成，真是令人痛心！
jingguo ji nian de nuli, yan kan jiu yao wancheng mubiao le, meixiangdao gong bai chui cheng, zhen shi ling ren tongxin!
After years of effort, the goal was almost reached, but then it failed. It's really heartbreaking!