A royal marriage could keep Japan aligned with Britain in 1940

How might a pre-First World War marriage between the British and Japanese royal families affect the Second World War?

It could pull Japan to Britain's side after France falls and bring Japanese naval aviation into the Mediterranean campaign.

For such a marriage to endure, Japanese court culture and Anglo-Japanese relations would already have changed substantially before the First World War. If the emperor's British mother became a respected public figure, Britain's isolation after the fall of France could create a powerful domestic case for intervention. Japanese policy in China might remain aggressive, but its navy could nevertheless fight beside Britain in Europe and the Mediterranean. Japanese carrier aircraft could follow the British night strike at Taranto with a dawn attack, potentially finishing the Italian fleet and forcing the United States to reconsider remaining outside the war.