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At the beginning of the war Japan had a decent fleet of submarines, and excellent torpedoes. Yet, they accomplished very little during the war due to operational doctrine that saw them as scouts and as weapons against warships. Eventually many of their larger/long range submarines ended up being impromptu cargo & troop carriers to many outlying garrisons as US subs decimated the Japanese merchant marine.

Let us assume (major hand wave here) that in return for some German aviators training in Japan for carrier ops on the planned German carriers, some Japanese submariners go to Germany to discuss submarine tactics with the U-boat veterans of WW1 and the new officers of the Kriegsmarine U-Boot Flott. All of this takes place BEFORE the outbreak of war, and the returning officers manage to convince the powers that be in the IJN that an anti-merchant campaign should be tried.

So, after Pearl Harbor you have a substantial number of Japanese submarines in place along the west coast of the US, the US-Hawaii route, the western approaches to the Panama Canal. Later subs will be dispatched along the Hawaii-Samoa-Australia axis. I predict the Japanese subs on the west coast will have a happy time even better than the Germans on the east coast - as they commence operations the afternoon of Dec 7 whereas the US had at least some time to get ready before the German U-boats arrived. After the first 4-6 months things get tougher for the Japanese, but ASW escorts and ASW air patrols needed for the Pacific over & above what went there OTL will either mean shifts in ship building and a/c building priorities, shifts in what we do have (more escorts go to the Pacific and 4 engine a/c that might have gone to England to bomb Europe are used for ASW patrols - the Navy used B-24s, why not B-17's esp along CA coast & off Panama).

IMHO this may slow things down a little in the Pacific, but diversion of effort from Europe will be necessary. Love to hear comments.
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