6th Fleet (Imperial Japanese Navy)
Despite these advantages, IJN submarines achieved remarkably little during World War II primarily having in hindsight suffered from the antiquated strategy of the Imperial Japanese Navy General Staff, who 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. Maybe the IJN General Staff was slow to change as the 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.
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.