Once the Lips Are Gone, the Teeth Will Feel Cold.
When two things are interdependent, the fall of one will endanger the other. According to the early chronicle Zuo’s Commentary on The Spring and Autumn Annals, when the State of Jin wanted to march through the State of Yu in order to attack Yu’s neighbor, the State of Guo. Gongzhiqi, a minister of Yu, remonstrated with his ruler, saying, “Guo provides a protective shield for Yu. If Guo falls, Yu will soon follow. The relationship between Yu and Guo is like that between lips and teeth.” This shows that since ancient times the Chinese nation has been keen to maintain friendly ties with neighboring countries. It represents pragmatic geopolitical thinking of maintaining amity with close neighbors.
To the states of Qi and Chu, the State of Zhao serves as a protective shield, just like the lips protecting the teeth. Once the lips are gone, the teeth will feel cold. If Zhao is defeated by the State of Qin today, the same fate will befall Qi and Chu tomorrow. (Records of the Historian)