The Second Sino-Japanese War and the Chinese Civil War were Proxy Wars from what I've observed. The United States, United Kingdom, and the Soviet Union supported the Second United Front while Nazi Germany and Fascist Italy supported Imperial Japan. The United States supported the Kuomintang while the Soviet Union supported the Chinese Communist Party. The POD I propose is, the Soviet Union refusing to support the Kuomintang at all. Instead, they fully support the Chinese Communist Party. Nazi Germany and Imperial Japan then fill the Soviet Union's vacancy as the allies Sun Yat-sen was looking for, creating a Communist Vs. Fascist Proxy War in China. The Invasion of Manchuria is thus diverted because Communism poses that much more of a threat to Imperial Japan than in OTL. The reason why I don't consider OTL to already be a Communist-Fascist proxy war of sorts is because the Soviet Union was helping both the Kuomintang and the Chinese Communist Party.