A Japanese declaration of war on Germany in 1939

How would Japanese forces be used if Japan declared war on Germany in 1939 and joined Britain?

Britain would welcome the Japanese fleet, whose Mediterranean deployment could deter Italy or make its naval position much worse.

Britain would welcome a Japanese declaration of war on Germany as a revival of their former partnership. Italy would then have to contemplate fighting the world's largest and third-largest fleets together. That could deter Italian entry into the war. If Italy still fought, Japanese reinforcements could greatly strengthen the Mediterranean fleet. A hypothetical Taranto attack illustrates the danger: a British night strike could be followed by a Japanese strike in daylight.