© 2007 Bob Hackett and Sander Kingsepp

SENSUIKAN!

Midget Submarines in Japan
and 'Operation Downfall' - 1945


1 March 1945:
The Second Special Attack Force (former First Special Base Unit) for Kaiten attacks is formed at Hikari, Yamaguchi Prefecture. Rear Admiral Nagai Mitsuru (former CO of JUNYO) is appointed CO. On that same day, a kaiten crew training unit is formed at Hirao, SE of Hikari.

20 March 1945:
The First Special Attack Force (planned strength 36 Kairyu type midget submarines and 8 Kaiten type human torpedoes) is formed on Jogashima island, Kanagawa Prefecture. Rear Admiral Obayashi Sueo (former CO of ZUIHO) is appointed CO. The Tenth Special Attack Force is formed at Kure and directly attached to Combined Fleet HQ. Rear Admiral Owada Noboru (former CO of YAMASHIRO) is appointed CO.

The Tenth Special Attack Force comprising submarine HA-109 and 12 Koryu midget submarines is tasked with developing new tactics for future Koryu and Kairyu attacks. In case of an Allied landing, it is to deploy to the landing area immediately to support the local special attack force in actions against the invasion fleet.

April to June 1945:
Submarines I-156, I-157, I-158, I-159 and I-162 are tasked to transport kaiten human torpedoes to various bases.

7 May 1945:
The Fourth Special Attack Unit (24 Kairyus and 4 Kaitens) is formed at Owase harbor, Mie Prefecture. Rear Admiral Mito Hisashi (former CO of KATORI) is appointed CO. His headquarters are located at Toba, Mie Prefecture, and his flagship is former submarine tender KOMAHASHI.

10 May 1945:
The Fifth Special Attack Unit is formed at Kagoshima, Kyushu. Rear Admiral Komazawa Katsumi (former CO of NISSHIN) is appointed CO.

20 May 1945:
The Third Special Attack Unit is formed at Sasebo for Koryu type midget and Shinyo exploding motor boat attacks. Rear Admiral Shibuya Kiyomi (former CO of NAGATO) is appointed the CO.

25 May 1945:
The Joint Chiefs of Staff issue a directive to five-star Generals of the Army Douglas MacArthur, Henry H. Arnold and Fleet Admiral Chester Nimitz to proceed with the invasion of Japan. [1][2]

World War II Joint Chiefs of Staff
(National Archives)

28 May 1945:
The Kaiten human torpedo is formally adopted into service.

Operation “Downfall” is the two-phase plan for the invasion of Japan. With the exception of a part of the British Pacific Fleet, "Operation Downfall" will be an American operation. On 1 November 1945, an amphibious assault, code-named Operation “Olympic”, will begin on the southernmost home island of Kyushu, followed about 1 March 1946 by Operation “Coronet”, the invasion of the main island of Honshu.

The naval armada for Operation Olympic will be the largest ever assembled. For the first time, the Third and Fifth Fleets will operate simultaneously. The Third Fleet, under Admiral (later Fleet Admiral) William F. Halsey, will be composed of 22 fleet carriers, 10 light carriers, 10 fast battleships plus cruisers and destroyers. Halsey's carriers will provide about 1,900 aircraft. The Fifth Fleet, under Admiral Raymond A. Spruance, will have over 20 carriers, 13 slow battleships, 20 cruisers plus destroyers and support ships or about 800 warships and about 1,500 transports will carry the invasion troops.

The Americans will assault Kyushu at three points. Four American Corps consisting of 11 Army and three Marine divisions of General Walter Krueger's VI Army are scheduled to take part in the initial landings. Once the southern third of the island has been seized for use as a staging point, naval and air bases will be established to support the next phase, Operation Coronet, the invasion of the Tokyo Plain.

General George C. Kenney's Far Eastern Air Force will commit 10 fighter groups, 14 bomb groups, three reconnaissance groups and three night fighter squadrons.

Operation Downfall, 1945-1946
(U.S. Army)

"Ketsu-Go"(Decisive Operation) is the Japanese plan for defeating the Allied invasion. Since the only beaches suitable for amphibious assaults are on Kyushu and the Kanto plain, south of Tokyo, Japanese intelligence correctly predicts the invasion will take place in southern Kyushu at Miyazaki, Ariake Bay and the Satsuma Peninsula.

The American landings at Kyushu will be opposed by 14 Japanese divisons, seven independent mixed brigades, three tank brigades and thousands of men of the Special Naval Landing Forces. These and other formations based on western Honshu and Shikoku come under the Second General Army commanded by Field Marshal Hata Shunroku, headquartered at Hiroshima Castle. On Kyushu alone, 790,000 mostly well dug-in Japanese with pre-registered weapons will oppose 550,000 American invaders.

Invasion Beaches on Kyushu

1 June 1945:
The Sixth Special Attack Unit is formed at Tanabe harbor, Wakayama Prefecture. Rear Admiral Yokoi Tadao (former CO of CHIYODA) is appointed CO.

21 June 1945:
At 1700, Okinawa is declared secure.

24 June 1945:
The 20th Air Force's 313th Bomb Wing mines and bombs Fukuoka, Karatsu, Sakai and Niigata harbors. Hikari Bay, Yamaguchi Prefecture. DesDiv 52's destroyer NASHI, converted into a kaiten carrier, is repeatedly near-missed by bombs or mines.

11 July 1945:
Hikari Bay. NASHI acts as a target for the kaiten-carrying submarine I-157.

19-25 July 1945:
Hikari Bay. I-36 conducts several kaiten training launches against NASHI operating as a target. On 25 July, a Kaiten piloted by Lt( j.g.) Wada Minoru is lost during training and Wada is killed.

July 1945:
The Japanese Army and Navy air forces have more than 12,500 aircraft of all types available to defend their homeland. More are to be available by the 1 November date set for the invasion. The Japanese plan to coordinate their kamikaze and conventional air strikes with attacks from their remaining conventional submarines, beginning when the invasion fleet is about 10 miles off Kyushu. The submarines will launch Kaiten suicide torpedoes to stop as many transports as possible.

About 2,000 IJA and IJN fighters will engage the Americans and gain air superiority over Kyushu. Another 300 or more IJN aircraft will attack the task force and keep it from protecting the troop transports. Meanwhile, another 800 or so kamikaze will crash their planes into the American transports. As the invasion armada grows closer, submarine attacks will intensify and another 2,000 kamikaze will attack in waves of 200 to 300. The Japanese estimate that the kamikazes will sink over 400 ships and perhaps destroy a third to a half of the invasion force before it lands.

KORYU Type D Midgets at Kure at War's End.
  (U.S. Navy Photo)

The IJN have few remaining warships left and little fuel. They still have some fleet submarines, but between April and June 1945, of 248 Kaiten suicide torpedoes ordered only 158 are produced. Instead of 300 smaller Kairyu submersibles, only 125 are built and instead of 110 Koryu midget submarines only 44 were produced.

In the case of the Kaitens and Kairyus, the Naval General Staff expects a 33 percent hit rate, 66 percent for Koryus and 10 per cent for Shinyo manned explosive motor boats. The five-man Koryu midget submarines will be employed with either two torpedoes or an explosive charge in the suicide role. The IJN plans to have 540 Koryu in service by the time of the invasion.

KAIRYU Hideaway
(National Archives)

The two-man Kairyu is armed with either two torpedoes or an explosive charge. Most Kairyu are based at Yokosuka and inlets on the southern tip of the Miura peninsula. By the fall of 1945, approximately 740 Kairyu are planned to be deployed for home island defense in concealed bases in Kyushu and Shikoku, but Allied bombing reduces considerably the number of midget submarines that will be available.

24 July 1945:
Potsdam, Germany, near Berlin. President Harry S. Truman, Soviet Premier Josef Stalin and British Prime Minister Clement Atlee attend the Potsdam Conference to discuss post-war Europe. Separately, Truman approves the plan for the invasion of Japan.

That same day, Rear Admiral J. Cary Jones, Jr.'s TG 35.3's USS PASADENA (CL-65), SPRINGFIELD (CL-66), ASTORIA (CL-90) and WILKES-BARRE (CL-103) and six destroyers from DesRon 53 conduct a high-speed anti-shipping sweep across Kii Suido channel. The Fourth Special Attack Unit at Owase is ordered to stand-by to intercept TG 53.3's cruisers.

25 July 1945:
Potsdam. The President orders the commencement of atomic attacks on Japan as soon as possible.

Kushimoto, Wakayama Prefecture. TG 35.3 shells an IJNAF seaplane base and airfield. Around 0600, twelve aircraft from TF 38 appear over Owase to attack targets of opportunity. Subsequent attacks continue until 1455. Between 0600 and 1457, KOMAHASHI and kaibokan CD-45 (the latter converted into a Shinyo type exploding motor boat tender) are subjected to successive air attacks and finally grounded off Owase. CD-45 loses 30 hands KIA and 19 wounded.

26 July 1945:
Potsdam. The Allies (United States, United Kingdom and China) broadcast the "Potsdam Declaration" that calls for the unconditional surrender of Japan, or the alternative of its prompt and "utter destruction". [3]

Jogashima Island. Around 0900, a lone B-24 (or a PB4Y Privateer) strafes the island and drops five bombs. One Kairyu of Rear Admiral Obayashi’s First Special Attack Force is slightly damaged by the strafing. 11th AF Consolidated B-24 “Liberators” attack the Kataoka naval base.

The First Special Attack Unit detachment stationed at Ajiro harbor, Shizuoka Prefecture (12 Kairyu type midget submarines) sustains loss of one submarine.

30 July 1945:
TF 38 aircraft conduct a sweep against Japanese shipping in Maizuru Bay. Ajiro harbor, Shizuoka Prefecture. Between 1100 and 1300 some sixty carrier planes raid the area. Three midget submarines of the First Special Attack Unit detachment (11 Kairyu type midgets) are damaged, one sailor is killed.

28 July 1945:
Toyko. Prime Minister Admiral, the Baron, Suzuki Kantaro(14)(former CO of TSUKUBA) announces that Japan will ignore the Potsdam Declaration.

30 July 1945:
TF 38 aircraft conduct a sweep against shipping in Maizuru Bay. Between 1100 and 1300, some sixty carrier planes raid Ajiro harbor, Shizuoka Prefecture. Three midget submarines of the First Special Attack Unit detachment (11 Kairyu type midgets) are damaged and one sailor is KIA.

Early August 1945:
The Shimoda detachment of the First Special Attack Force (12 Kairyu type midgets) receives a report about the sighting of an American submarine shelling Mikimoto lighthouse, off Shimoda harbor. A Kairyu is diespatched to intercept the submarine, but fails to locate it.

6 August 1945:
At 0815, Colonel (later Brig Gen) Paul W. Tibbetts’ B-29 "Superfortress", nicknamed "ENOLA GAY", of the 509th Composite Group, drops the 15-kiloton yield "Little Boy" uranium atomic bomb on Hiroshima.

Colonel Tibbetts with B-29 ENOLA GAY

That same day, following TG 35.3's bombardment of Kushimoto, four Kaitens are deployed from Otsujima base to Tanabe to be attached to the Sixth Special Attack Unit.

8 August 1945:
Moscow declares that from 9 August 1945, the Soviet Government will consider itself to be at war with Japan.

9 August 1945:
At 1101, Major (later Brig Gen, ANG) Charles W. Sweeney's B-29 "BOCKSCAR", of the 509th Composite Group's 393rd Bomb Squadron, drops the 21-kiloton yield "Fat Man" plutonium atomic bomb, on Nagasaki. [4]

That same day, carrying out Stalin's pledge at Yalta, Marshal Aleksandr Vasilevsky, CINC, Soviet Far East Forces, launches Operation "August Storm", the invasion of Japanese-held Manchuria (Manchukuo). The attack is made by three Soviet army groups ("fronts") comprising 80 divisions of 1.5 million men. In less than two weeks, the Soviets defeat General Yamada Otsuzo's depleted and ill-equipped Kwantung Army of over 600,000 men. [5]

10 August 1945:
Japan offers to surrender to the Allies, if Emperor Hirohito (Showa) is allowed to remain the nominal head of state.

12 August 1945:
The United States announces it will accept the Japanese surrender and that the emperor can remain in a ceremonial capacity.

Shikoku, Kochi Prefecture. That same evening, the Suzaki kaiten detachment of the Eighth Special Attack Unit receives a report about the sighting of an enemy task force off Shionomisaki, Wakayama Prefecture. Based on that information the local IJA commander expects a landing at Tosa Bay the next morning. Two kaitens are immediately dispatched to Tosa Bay and sortie at 0600 the next morning, but fail to locate the enemy and return by 1000.

13 August 1945:
Tokyo. At an evening conference attended by General Umezu Yoshijiro, Chief of the Army General Staff and Admiral Toyoda Soemu (33), (former CO of HYUGA), Chief of the Navy General Staff , the Vice Chief of the NGS, wild-eyed Vice Admiral Onishi Takijiro (40)(former XO of KAGA) proposes "that if we are willing to sacrifice 20 million Japanese lives in special attacks (kamikaze), victory can still be achieved!"

14 August 1945:
Tokyo. At 1020, the emperor convenes a conference of his most senior military officers. Field Marshall Hata, freshly arrived from Hiroshima, expresses no confidence in Japan continuing the war over appeals from such strong-willed, arrogant personalities as Field Marshal Sugiyama Hajime and Fleet Admiral Nagano Osami who exhibit a dull-witted state of denial. The emperor dismisses their protestations for protracted carnage.

The emperor notes that with the Soviet entry into the Pacific War and the enemy's use of atomic weapons, not even Onishi's Special Attack forces can stop them. He requests that his senior officers cooperate with him to end the war. Later, the Japanese announce that the emperor has decided to accept the Potsdam Declaration's terms and end the War, effective the following day.

That same day, 167 B-29s of the 20th Air Force from Saipan bomb Hikari Naval Arsenal, Yamaguchi Prefecture. The raid is supported by North American P-51 “Mustang” fighters from Iwo Jima, attacking various targets in the same area until 1040 in the morning. 71.8 percent of the arsenal’s total roof area is destroyed. 738 workers, mostly mobilized middle school students, die in the attacks.

Emperor Hirohito Reads an Imperial Rescript

15 August 1945: Cessation of Hostilities:
Imperial Palace, Tokyo. At noon, the emperor announces Japan's surrender that is broadcast by radio all over the Japanese Empire.

Port Arthur, Manchuria. Lost to Japan in 1905, the Soviet Navy Flag flies again on 22 August

18 August 1945:
The Red Army makes three amphibious landings in northern Korea, one landing in Sakhalin and one at Shimushu, Kurile Islands. Korea is subsequently divided at the 38th parallel into Soviet and U.S. zones. After the surrender of the Kwantung Army in Manchuria, about 590,830 Japanese are sent to PoW camps in Siberia. [6]

Surrender
(National Archives)

2 September 1945: Formal Surrender Ceremonies:
Tokyo Bay. Japanese representatives arrive on board USS MISSOURI (BB-63) more than two weeks after Japan accepted the Allies' terms. Foreign Minister Shigemitsu Mamoru and General Umezu Yoshijiro, Chief of the Army General Staff are in the front row. Witnessing the surrender on behalf of the IJN are Rear Admiral, the Baron, Tomioka Sadatoshi (former CO of OYODO)(middle row, behind Umezu), Rear Admiral Yokoyama Ichiro (former CO of KUMA)(back row, right) and Captain Shiba Katsuo (former CO of OI) (back row, behind Tomioka). The others present are the witnesses of the IJA and the Foreign Ministry. At about 0905, Shigemitsu and Umezu sign the Instrument of Surrender, followed by General of the Army, Douglas MacArthur, Supreme Commander for the Allied Forces, Fleet Admiral Chester A. Nimitz, representing the United States, and eight representatives of the Allied Forces.

At the time of surrender, 43 Kaiten torpedoes (mostly Type 1 or Type 1 Improved) are located at Otsujima base, 52 at Hikari, 39 at Hirao, 37 at Kure and 16 at Oga. All are destroyed or scuttled after the end of hostilities. 224 Kairyu submersibles were built by the end of war plus another 207 were in various stages of completion. In addition to about 115 Koryus built during the war, some 500 are found incomplete.

With the War's end, Operation “Downfall” is cancelled. General of the Army George C. Marshall, CoS, U.S. Army, estimates up to one million casualties for the invasion of Japan, but a contemporary Army study estimates the invasion and subsequent Battle for Japan would have cost as many as four million American casualties, including up to 800,000 fatalities, and five to ten million Japanese fatalities.

If Downfall had become necessary and American forces were contested heavily in southern Japan, the Soviets most probably would have secured Hokkaido and invaded northern Honshu. This may have led to the vanquished nation's partitioning into North and South Japan, as occurred in Korea and persists to this day.

May 1978:
During the dredging works at Ajiro harbor, Shizuoka Prefecture, the wreck of a scuttled (or sunken) Kairyu type midget submarine is discovered. After restoration it is now displayed at Arashiyama art museum in Kyoto.

August 1989:
The well-preserved hull of a Type 4 Kaiten is discovered under one of the buildings of the Takeda Pharmaceutical Co. Ltd. located on the premises of the former Hikari Naval Arsenal.


Authors' Notes:
[1] The Joint Chiefs of Staff, shown in 1943, are from left to right: Gen Henry H. Arnold, CoS, U.S. Army Air Forces; Adm William D. Leahy, Chairman, JCS; Adm Ernest J. King, Chief of Naval Operations and Gen George C. Marshall, CoS, U.S. Army.

[2] Gen Arnold, in addition to being CoS of the Army Air Forces and a member of the JCS, was the self-appointed commanding general of the USAAF's Twentieth Air Force (B-29s).

[3] The Soviet Union was not a signatory to the Potsdam Declaration as they were not at war with Japan at the time.

[4] Sweeney's primary target was the port city of Kokura on Kyushu; however, it was obscured by smoke and haze, so Sweeney proceeded on to his secondary target, Nagasaki.

[5] At the Yalta Conference, held 4-11 February 1945 in the Crimea (now in Ukraine), between the US, UK and the Soviet Union, Premier Stalin agreed to attack Japan within 90 days after the defeat of Germany. After the defeat of Japan, the Kurile Islands and the southern part of Sakhalin ceded to Japan by the Treaty of Portsmouth after the 1904-1905 Russo-Japanese War were returned to the Soviet Union as agreed at Yalta.

[6] By 1950, 510,409 Japanese POWs were repatriated to Japan by the Soviets.

Thanks for assistance go to Dr. Higuchi Tatsuhiro of Japan.

-Bob Hackett and Sander Kingsepp


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