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).
Japanese submarine I-55 (1925)
Following World War I, the Imperial Japanese Navy re-evaluated the use submarine warfare as an element of fleet strategy due to the successful deployment of long-range cruiser-submarines for commerce raiding by the major combatant navies. Japanese strategists came to realize possibilities for using the weapon for long range reconnaissance, and in a war of attrition against an enemy fleet approaching Japan. Two large, long-range Japanese submarines had already been built under the Eight-six fleet program as prototype ( and ), however, the arrival on 20 June 1919 of seven German U-boats received by Japan as war reparations at the end of World War I led to a complete re-design. The Japanese quickly hired hundreds of German submarine engineers, technicians and former U-boat officers unemployed by the defeat of Germany in World War I, and brought them to Japan under 5-year contracts. The American ONI (Office of Naval Intelligence) estimated that some 800 German advisors had gone to Japan by the end of 1920. The Japanese also sent delegations to Germany, and were active in purchasing many patents.