假道伐虢
jiǎ​dào​fá​guó

to conquer Guo by way of Yu

假道伐虢 jiǎ​dào​fá​guó
to conquer Guo by way of Yu
Borrowing the resources or passage of an ally to attack a common enemy, then turning on the ally; an idiom based on a story from the Spring and Autumn Period where the State of Jin asked for passage through the State of Yu to attack the State of Guo, only to conquer Yu on the way back

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