Borrow the Road to Conquer Guo
Borrow the resources of an ally to attack a common enemy. Once the enemy is defeated, use those resources to turn on the ally that lent you them in the first place.
Spring and Autumn Period China
The small states of Yu and Guo bordered the larger state of Jin. Duke Xian of Jin desired to conquer both states. This desire was not unknown to the two smaller states and both had taken steps to defend their borders with Jin. The duke's general, Xun Xi, suggested they make a roundabout attack at Guo through the state of Yu to catch them by surprise. General Xun suggested that since the duke of Yu was a greedy man he could be bribed with gifts of jade and horses in exchange for safe passage through his territory. Duke Xian objected to the idea of giving away so much treasure and asked: "What if the duke of Yu accepts our gifts but refuses us passage?" but general Xun replied: "If he doesn't intend to let us through, then he wouldn't accept them, but if he does accept the gifts, and he does let us through, then it will only mean that the treasure is stored temporarily in his storehouse rather than ours." When the bribe was sent to the duke of Yu one of his ministers, Gong Ziqi, cautioned against accepting them saying: "Yu is to Guo, like lips are to teeth. Our ancestors had a saying; `If the lips are gone, the teeth will be exposed to cold'. That Guo is able to exist depends on Yu while Yu's ability to survive hinges on Guo. If we make way for Jin, then the day will see Guo perish in the morning to be followed by Yu in the evening. Why should we ever let Jin pass?" The duke of Yu, however, refused to listen to this advice. Jin was given safe passage and succeeded in conquering Guo. On their way back they stopped to conquered Yu. After taking the Yu capital and recovering the treasure, general Xun returned the jade and horses to the duke. Duke Xian was pleased and said in good humor "The jade is untouched but the horses seem to have gained some more teeth!"