Odd thought I had, probably inspired by all those ASB threads that swap Japanese and German military stuff in WW2.
Japan did deploy ships to the Med in WW1 when it was on the Entente side, so there is a precedent there.
One would assume that this would have to come at a time when Japan wasn't fighting, or planning to fight, in the Pacific.
How about this: Japan sends a fleet to European waters in 1938 to do joint exercises with Germany to cement an earlier Tripartite Pact-style alliance (I think Hendryk had a thread about such) and due to butterflies and escalation WW2 breaks out earlier, perhaps over the Sudetenland. Japan, in an awkward position, initially tries to remain neutral but a British attack on the Kriegsmarine ends up catching the IJN force in the crossfire and war breaks out, with the Japanese government viewing this as coming at an inconvenient time but still taking the opportunity to go after Malaya and Indochina. Perhaps the Germans promise them the Dutch East Indies in exchange for immediate naval assistance...
What about other possible scenarios to get this?