One peculiar aspect of the Imperial Japanese Navy's submarine force was it's widespread use of aircraft launched from submarines. Other navies experimented with this, but only the Japanese pursued it with such energy. These float planes expanded the scouting range of the sub, and in theory allowed them to reconnoiter enemy ports. One was even used to carry out the only aerial bombing of the continental United States in World War Two near Brookings, Oregon in September 1942.
If the Japanese had ever turned their subs completely loose on shipping they would have been quite an advantage. Sure beats that tethered helicopter the Germans experimented with.
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