Japan stomps hard on the first (few) assassinations, decrying the assassins as unpatriotic, unJapanese, etc.
Thus, they stay out of the quagmire that is the Chinese war. (Although they probably have Manchuria.)
When WWII approaches, China cozies up to Germany (somewhat more than OTL), and the Japanese are only a minor irritant to the US. (The China lobby isn't annoyed by the Japanese invasion of the main part of China, and the German connexion means the rest of the US isn't as interested in listening to the China lobby, anyway.)
Moreover, when Nazi-allied China tries taking over the foreign enclaves in the various port cities, the European powers can't spare the forces to defend them, so the US (partly) and Japan (mostly) provide the defences.
While Japan may not be fully a member of the Allies (perhaps), it's definitely on their side, as opposed to on the Nazi's side.
OTOH, the only 'Pacific War' is seizing Vichy Indochina, and 'protecting' the treaty ports in China.
only quibble (or question) would be whether or not French might hang on to Indochina. IOTL they were supplying China prompting (among other reasons) Japanese invasion. ITTL there would be no instructions from Germany for Vichy regime to cooperate with Japan, and assume the U.S. is not at war with France?
wonder the u-boat operations in Pacific and Indian Oceans?
China had ordered u-boats pre-war, not delivered at Japan's request.