The Second Chinese Civil War (2016 - 2019) was fought between the Communist loyalists, Tibetan nationalists and the Chinese Nationalist rebels. It was fought during the Zombie War and was conventional up until 2019 when the Communist Politburo initiated a series of tactical nuclear strikes on Nationalist strongholds. What the Chinese Premier didn't know was by this time, nearly half of all Chinese nuclear weapons were in control of the Nationalists. This broke the back of both sides, leading to an armstice of between a rump PRC located in Southern China and the new United Republic of China controlling the rest of China Proper. Tibet also declared independence in early-2017 and successfully repelled any PRC attempt to retake Tibet. The Communists then decided to focus on the greater Nationalist threat and left the Tibetans alone for the time being.
Due to the Mainland Nationalists owning Northern China and the People's Republic of China preparing for a Taiwanese Invasion, Japan stepped in and occupied the island, stopping the Communists from invading.
Xinjiang, Mongolia, and Manchuria are all depopulated, but are being looked at by the Holy Russian Empire, something the Chinese nations are powerless to do anything about due to their already pre-existing determination to reunite the two Chinese nations.