Submarines of the Imperial Japanese Navy
Imperial Japanese Navy (IJN) submarines formed by far the most varied fleet of submarines of World War II, including manned torpedoes ("Kaiten"), midget submarines ("Kō-hyōteki", ), medium-range submarines, purpose-built supply submarines (many used by the Imperial Japanese Army, see Type 3), fleet submarines (many of which carried an aircraft), submarines with the highest submerged speeds of the conflict ("Sentaka" ), and submarines able to carry multiple bombers (World War II's largest submarine, the Sentoku ). They were also equipped with the most advanced torpedo of the conflict, the oxygen-fuelled Type 95 (which are sometimes confused with the famed Type 93 Long Lance torpedo).
Allied submarines in the Pacific War
From 1943, Allied submarines waged an increasingly effective campaign against Japanese merchant shipping and the IJN. By the end of the war in August 1945, the Japanese merchant marine had less than a quarter of the tonnage it had in December 1941. Overall, U.S. Navy submarines sank around 1,300 Japanese merchant ships, as well as roughly 200 warships. Despite the need to maintain sea lanes for its empire, the Japanese never successfully developed a cost-effective destroyer escort better suited for convoy duties, while it also did not have the industrial might to replace the losses of its heavily armed destroyers, nor of its ill-protected merchantmen.