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During the Second World War, the Imperial Japanese Navy fielded a fairly large and quite capable submarine force, then wasted it via a doctrine which emphasized attacks on naval vessels rather than on merchant shipping. As a result, the IJN's subs accomplished very little during the war.

But what if, following World War One, the IJN had looked at the achievements of Germany's submarine force and come to the conclusion that attacking merchant shipping was the proper role of a submarine? Let's say they devote a bit more of their resources to submarine construction...maybe by building one less battleship of the Yamato class...and adopt a doctrine similar to that of the Germans for the use of their submarines. During the war, Japanese subs prowl the Pacific and Indian Oceans, hunting merchantmen in the Gulf of Aden, in the Malagasy Straits, the approaches to Australia and New Zealand, off the coast of India, and on the western side of the Canal Zone. How does this affect the war?

Some immediate possible effects...

--The Germans don't have to deploy any subs in the Indian Ocean, as Japanese boats will be operating there. This provides a few more subs for the Atlantic War.

--A greater appreciation by the Japanese of the possible impact of submarine warfare on merchant shipping leads Japan to develop effective ASW technology sooner. Greater losses for American submarines, and more supplies get through to Japan and it's island outposts.

--America finds it more difficult to shift men and supplies to the Pacific via the Canal. With what effects? Maybe someone more knowledgable on that aspect of Pacific operations could expound.

--Greater losses to allied shipping going to Britain from India and other eastern ports. This contributes to the German effort in the Atlantic War.

Any other thoughts?
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