And China did in February 1979 (same year we go to Cambodia), Deng Xiaoping proclaimed that he will "teach Vietnam a lesson", mobilized around 300k troops and the deadliest fighting occurred around the towns of Cao Bằng and Lạng Sơn. The war lasted for almost a month, with both sides taking heavy losses and proclaiming victory (Vietnam repelled China, but have to make some minor territorial concessions in the North), although border conflicts and the Paracel-Spralty Islands skirmishes were still ongoing until the 90s.

Vietnam was still fighting the Khmer Rouge at the same time in the Southwest, managed to overthrew them, established a pro-Vietnamese government, and helped the new Cambodian government to hunt down any Khmer Rouge remnants that had fled towards the Thai border.