假道伐虢

jiǎ dào fá guó

to obtain safe passage to conquer the State of Guo

to borrow the resources of an ally to attack a common enemy (idiom)

jiǎ

variant of 假[jia3]

dào

road; path (CL:條|条[tiao2],股[gu3])

to cut down

guó

surname Guo