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.
History of submarines
After the attack on Pearl Harbor, many of the U.S. Navy's front-line Pacific Fleet surface ships were destroyed or severely damaged. The submarines survived the attack and carried the war to the enemy. Lacking support vessels, the submarines were asked to independently hunt and destroy Japanese ships and submarines. They did so very effectively.