反戈一击 Turn against
Explanation
比喻掉转武器，攻击原先所属的阵营。也指背叛原来所属的集团，转而投向对方。
A metaphor for turning one's weapon and attacking one's original camp. It also refers to betraying one's original group and switching to the opposing side.
Origin Story
话说三国时期，蜀汉名将魏延，因性格刚烈，屡次与诸葛亮产生矛盾。诸葛亮临终前，特意安排马岱诛杀魏延，以绝后患。魏延不服，率军反叛，结果在诸葛亮事先安排好的埋伏下，被马岱率兵击败，最终身首异处，成为反戈一击的典型案例。这一事件也反映出蜀汉内部的权力斗争激烈，以及诸葛亮对蜀汉未来稳定发展所做的周全考虑。魏延反戈一击，最终失败，也为后世留下了深刻的教训：投机取巧，背叛他人，最终必将自食恶果。而诸葛亮则以其远见卓识，为后世留下了许多治国理政的宝贵经验，至今仍值得人们学习和借鉴。
During the Three Kingdoms period of ancient China, there was a conflict between the Shu Han general Wei Yan and Prime Minister Zhuge Liang. Zhuge Liang sentenced Wei Yan to death, but Wei Yan disagreed and began a rebellion. During the battle, General Ma Dai attacked Wei Yan. Wei Yan was defeated and killed. This story illustrates the consequences of turning against one's former allies.
Usage
多用于军事战争等方面，形容突然改变目标，向原先的阵营发动攻击。
Mostly used in the context of military warfare, to describe suddenly changing the target and attacking the original group.
Examples
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他临阵倒戈，反戈一击，令人震惊。
ta linzhen daoge,fange yiji,ling ren zhenjing.
He defected and attacked his former allies.
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敌军将领突然反戈一击，改变了战局的走向。
dijunjiangling tu ran fange yiji,gaibianle zhanju de zouxiang.
The enemy commander suddenly turned against his own troops, changing the course of the battle.