6th Fleet (Imperial Japanese Navy)

The IJN 6th Fleet (第六艦隊 (日本海軍)) 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 lanes 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 submarines 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 ships. In 1942, Japanese submarines were credited with sinking two aircraft carriers, one cruiser and several destroyers, as well as damaging one aircraft carrier and two battleships.

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.

15 November 1940

 * Submarine Squadron 1 : Training cruiser Katori (Flagship), Submarine tender Taigei, Submarine I-20
 * Submarine Flotilla 1 : Submarine I-15, I-16, I-17
 * Submarine Squadron 2 : Light cruiser Isuzu
 * Submarine Flotilla 11 : Submarine I-74, I-75
 * Submarine Flotilla 12 : Submarine I-68, I-69, I-70
 * Submarine Flotilla 20 : Submarine I-71, I-72, I-73
 * Submarine Squadron 3 : Submarine tender Chōgei, Submarine I-7
 * Submarine Flotilla 7 : Submarine I-1, I-2, I-3
 * Submarine Flotilla 8 : Submarine I-4, I-5, I-6

1 December 1941

 * Training cruiser Katori (Flagship), Fleet oiler Ondo
 * Submarine Squadron 1 : Auxiliary submarine tender Yasukuni-maru, Submarine I-9
 * Submarine Flotilla 1 : Submarine I-15, I-16, I-17
 * Submarine Flotilla 2 : Submarine I-18, I-19, I-20
 * Submarine Flotilla 3 : Submarine I-21, I-22, I-23
 * Submarine Flotilla 4 : Submarine I-24, I-25, I-26
 * Submarine Squadron 2 : Auxiliary submarine tender Santos-maru, Submarine I-7, I-10
 * Submarine Flotilla 7 : Submarine I-1, I-2, I-3
 * Submarine Flotilla 8 : Submarine I-4, I-5, I-6
 * Submarine Squadron 3 : Submarine tender Taigei, Submarine I-8
 * Submarine Flotilla 11 : Submarine I-74, I-75
 * Submarine Flotilla 12 : Submarine I-68, I-69, I-70
 * Submarine Flotilla 20 : Submarine I-71, I-72, I-73

15 June 1944
(The 6th fleet does not have a flagship, because the headquarters moved to the land.)
 * Submarine I-10
 * Submarine Squadron 7
 * Submarine Flotilla 51 : Submarine Ro-104, Ro-105, Ro-106, Ro-107, Ro-108, Ro-109, Ro-111, Ro-112, Ro-113, Ro-114, Ro-115, Ro-116, Ro-117
 * Submarine Squadron 8 : Submarine I-8, I-26, I-27, I-29, I-37, I-52, I-165, I-166, Ro-501
 * Submarine Squadron 11 : Submarine tender Chōgei, Submarine I-33, I-46, I-54, I-55, I-361, I-362, Ro-46, Ro-48
 * Submarine Flotilla 7 : Submarine I-5, I-6
 * Submarine Flotilla 12 : Submarine I-169, I-174, I-175, I-176
 * Submarine Flotilla 15 : Submarine I-16, I-32, I-36, I-38, I-41, I-44, I-45, I-53
 * Submarine Flotilla 22 : Submarine I-177, I-180, I-183, I-184, I-185
 * Submarine Flotilla 34 : Submarine Ro-36, Ro-41, Ro-42, Ro-43, Ro-44, Ro-45, Ro-47

1 June 1945

 * Patrol Squadron 22 : Auxiliary boom defence vessel Kiku-maru
 * 4 Patrol Divisions
 * Hunter-Killer Squadron 31 : Destroyer Hanazuki
 * Destroyer Division 17 : Destroyer Yukikaze
 * Destroyer Division 41 : Destroyer Suzutsuki, Fuyuzuki
 * Destroyer Division 43 : Destroyer Take, Kiri, Maki, Sii
 * Destroyer Division 52 : Destroyer Sugi, Kashi, Kaba, Escort ship CD-31, CD-43
 * Submarine Squadron 11 : Submarine tender Chōgei, Submarine I-201, I-202, I-203
 * Submarine Flotilla 1 : Submarine I-13, I-400, I-401
 * Submarine Flotilla 15 : Submarine I-36, I-47, I-53, I-58
 * Submarine Flotilla 16 : Submarine I-369, I-372, Ha-101, Ha-102, Ha-104
 * Submarine Flotilla 34 : Submarine Ha-109

Commanders of the IJN 6th Fleet
Commander in chief

Chief of staff