A very public back and forth between South Korea and China over the deployment of an American missile defense system erupted this week, sowing the seeds for another dramatic fallout between Seoul and Beijing.

On Wednesday, Chinese foreign ministry spokesperson Zhao Lijian urged South Korea to stick to the previous government’s promise in 2017 not to deploy any more U.S. Terminal High Altitude Area Defense (THAAD) missile interceptor systems. At the time, Beijing complained that THAAD’s radar systemcould be used to track the movement of China’s missile forces, even though the deployment came in response to North Korean missile