How Allied Japan might deter Italy in 1940

Assuming no Second Sino-Japanese War, if Japan joined Britain and France against Germany, would Italy still enter on Germany's side?

Possibly not; combined British-Japanese naval strength could make intervention too risky for Mussolini.

The Regia Marina facing both British and Japanese naval power would make an opportunistic Italian entry much less attractive. A Japanese fleet in the Mediterranean would change the local balance before Italy chose whether to join Germany. Mussolini also lacked Hitler's degree of control over his government and could be removed when defeat became visible. The counterfactual therefore turns on both naval deterrence and the political risk of backing an apparently losing side.