6th Fleet ( Imperial Japanese Navy ) The was a fleet of the Imperial Japanese Navy during World War II , primarily responsible for command of submarine operations . History . The IJN 6th Fleet was formed on 15 November 1940 , and was assigned general control of all Japanese submarine operations . Its initial mission was reconnaissance off the west coast of the United States , east coast of Australia , and the sea lane s of the Indian Ocean . Background . Prior to World War II , Japan had a very diverse submarine fleet , including the only submarines of over 5 , 000 tons submerged displacement , or submarines over 400 feet in length until the advent of nuclear power . Japan was also unique in that it built 41 submarines that could carry aircraft : something no other nation had yet produced . Japan also had submarines with the longest ranges and highest speeds of any nation . With the development of the Type 95 submarine - launched variant of the Long Lance oxygen - propelled torpedo , Japan not only had the world ' s most advanced torpedo , but one with the largest warhead . Despite some advantages , Japanese submarines were generally slow to dive , easy to track with radar and sonar , difficult to maneuver underwater , and less sturdy than their German U - boat counterparts . They achieved remarkably little during World War II , primarily due to the antiquated strategy of the Imperial Japanese Navy General Staff , which viewed submarines as little more than scouts whose main role was to locate and shadow enemy naval task forces in preparation for a decisive surface conflict . Early stages of the Pacific War . At the start of the Pacific War , Midget submarine s were used in preparatory reconnaissance of the US Navy anchorage at Pearl Harbor , Hawaii , and in the initial stages of the attack on Pearl Harbor . With the outbreak of general war , the mission of the IJN 6th Fleet expanded to include shipping interdiction and mine laying . A few specialized missions , such as the Attack on Sydney Harbour with the use of midget submarines were also undertaken . The IJN 6th fleet cooperated briefly with the German Navy in the Indian Ocean in operations to interdict British commerce from its base in Penang , but these missions were a minority . The Navy General Staff placed more emphasis on ambush operations of Allied capital ship s . In 1942 , Japanese submarines were credited with sinking two aircraft carrier s , one cruiser and several destroyer s , as well as damaging one aircraft carrier and two battleship s . However , the success of 1942 could not be repeated in 1943 , due to increased anti - submarine capabilities by the US Navy and to the vast scale of the Pacific conflict , which made it difficult for a submarine to be in “ right place at the right time ” to make a successful attack . Latter stages of the Pacific War . After 1942 , the Navy General Staff also gave very little support to continued commerce interdiction or ambush operations . Instead , submarines came to be increasingly used ( especially after massive Japanese surface vessel losses in the Solomon Islands campaign for transport of critical supplies and reinforcements to the starving garrisons on isolated islands . The Japanese Navy expended hundreds of sorties on such missions , which might have otherwise been used offensively against the Allied war effort . In the meantime , continuous developments in anti - submarine warfare by the US Navy resulted in ever - increasing losses for Japan ' s submarine fleet . Japan started the war with 63 ocean - going submarines ( not including midgets ) , and completed 111 during the war , for a total of 174 . Of this total , 128 vessels were lost during the conflict . Most of the survivors were either training vessels , or were else recently completed by the end of the war and never saw combat . Of the 30 submarines that supported the Pearl Harbor attack , none survived the war . Kwajalein in the Marshall Islands was the main base for Japanese submarine operations in the Pacific until it fell to the Americans in February 1944 . IJN 6th fleet headquarters was relocated to Saipan in the Mariana Islands , which the fell to the Americans in July 1944 with the death of IJN 6th fleet commander in chief Admiral Takeo Takagi and most of his staff . The remnants of the IJN 6th fleet were based at Ulithi Atoll in the Caroline Islands until the end of the war . Japan ' s submarine forces were unable to provide much support during the Battle of the Philippines due to increased vigilance by the American fleet . In the final stages of the war , surviving submarines were largely used as carriers for " kaiten " suicide missions . The final sortie of the IJN 6th fleet was after the termination of the war , when the super submarine Japanese submarine I - 401 returned to Yokosuka from Ulithi . Commanders of the IJN 6th Fleet . Commander in chief Chief of staff