An unknown fact is that the Russian government, while the UK and the US watch, did assist China to fight the invasion.
Peter Harmsen tells us that 26 Russian Tupolev SB bombers took off shortly after New Year 1938 from Wuhan to bomb a Japanese air base near Nanjing.
48 Japanese planes were destroyed on the ground. The attack at dawn was a complete surprise. [p 67 Storm Clouds over the Pacific 1931-1941, 2023 ed]
A few weeks later 28 Russian Tupolev SB bombers took off to bomb Japanese military airport in Formosa. The attack was again successful.