From 《
左传·
僖公五年》 (722-468 BC): the state of Jin (
晋) asked Yu (
虞) for safe passage to attack Guo (
虢). Once Guo fell, Jin's army turned around and conquered Yu on the way home. The cautionary partner idiom
唇亡齿寒 (chúnwángchǐhán, 'when the lips are gone, the teeth get cold') comes from the same episode.