© 2013-2016 David Dickson, Bob Hackett and Sander Kingsepp
Revision 5

(USS INDIANA at Majuro after collison with USS WASHINGTON, Feb '44)
(Offical U.S. Navy Photo, NARA via Naval Historical Center)


By David Dickson, Bob Hackett and Sander Kingsepp


May 21, 1944: A.P.; In what some are calling a second Pearl Harbor . . .

Imagine that headline in the U. S. newspapers in May 1944. Such a headline was something the Japanese actually were planning. In February 1944, Japanese long range reconnaissance, primarily from Nauru and Ocean, established that the Americans had developed large advance fleet bases at Eniwetok, Kwajalein and Majuro. Majuro was the main base for the Fast Carrier Force. The attack was inspired by the USMC commando raid on Makin. The Japanese had planned to raid Espiritu Santo in similar fashion in October 1942, but the idea was abandoned after submarine reconnaissance revealed the island was too heavily guarded.

The obvious model for a surprise attack on Majuro was Pearl Harbor. Its lessons were apparent in the new Japanese plan, except, of course, a daylight attack was now impracticable. The attack was first scheduled to 10-11 April, but later was postponed until 20-21 May.

The basic outline of the plan was taking form. In early May, the First Mobile Force's (Dai Ichi Kido Butai) nine carriers with their screening ships, plus four escort carriers with a total of 530 aircraft embarked would depart the Inland Sea passing the Bonins, Marcus and Wake to a point NE of Majuro. There on the night of 20-21 May 1944, the IJN would launch a surprise attack. 20-21 May was chosen because it was a night of good moonlight. At the same time, 300 land based aircraft would be shuttled in through Marcus, Wake, Truk and Ponape to assist the carrier-borne aircraft. Lastly, five submarines equipped with ten aerial torpedo equipped amphibious light tanks would arrive off the atoll. The tanks would be launched outside the atoll, cross the coral reef and attack the carriers in the anchorage. A submarine picket line would protect the eastern flank of Mobile Force on its final run-in.

Captain (later Rear Admiral) Yamamoto Chikao, Chief of Operations Branch of the Naval General Staff, had been the officer primarily responsible for developing the plan and now Admiral Shimada, Chief of the Naval Staff approved it. Yamamoto, accompanied by a small staff, left Yokohama during the last week of March and flew to the Island of Palau in the western Carolines, advance base for the Japanese Fleet, for a conference with Admiral Koga Mineichi, Commander Combined Fleet. The meeting took place on the night of 26-27 March 1944 in the flag plot of the battleship MUSASHI. As the plan was outlined to Admiral Koga and several of his staff, some voiced serious misgivings stating the operation should be deferred until further studies could be completed. Yamamoto persisted and won the support of the majority of the staff, including Koga. The balance of the night was spent refining the basic outline. The night of 20-21 May was selected as the time for the attack and the plan, as previously presented to Admiral Shimada, was adopted.

The next morning Yamamoto, with his staff, left MUSASHI, went ashore, boarded their aircraft and with overnight stops at Manila and Takao, Formosa returned to Tokyo arriving on 29 March. Two days later Admiral Koga and his staff left Palau by flying boat. Koga’s plane crashed en route, killing the Admiral and most of his staff. YU-GO Sakusen (Operation "Supremacy”) as the plan was now designated, was shelved pending appointment of a new Commander of the Combined Fleet. For a month, the Combined Fleet was in the hands of a caretaker commander and “YU” lay dormant.

The centerpiece of the planned Japanese attack was to be the Mobile Force that later fought the Battle of the Philippine Sea, reinforced by four auxiliary carriers. *1

The second component of the attack was to be the Base Air Forces, the Japanese Navy’s shore based air component. The approximate forces available to Base Air Force for “YU” would be 60 land based fighters, 40 land based bombers, 100 carrier fighters, 50 carrier bombers and two shore based reconnaissance aircraft of the First Base Air Force together with twenty land based fighters of 22nd Air Flotilla, totaling 272 aircraft.

The submarine-related part of YU-GO was called "Tatsumaki Sakusen” (Operation Tornado). The submarine launched torpedo carrying light amphibious tank attack is the most interesting - or bizarre - component of the operation. The plan was for five cruiser submarines; I-36, I-38, I-41, I-44 and I-53, each equipped to carry two Type 4 "Ka-Tsu" amphibious tanks on its afterdeck.

The 16-ton Type 4 amphibious tanks carried Type 2 (450-mm) surface-launched torpedoes with a 350-kg Type 97 explosive warhead. Type 2 torpedoes were usually launched from torpedo boats. The Type 2 was a simplified variant of Type 91 Mod. 3 aerial torpedo. The small 17-foot tank’s 62-horsepower gasoline engine was fitted with a watertight pressure box. The plan called for the tanks to approach the atoll, swim ashore, cross the land, enter the lagoon and attack the anchored American carriers. *2

"Ka-Tsu" amphibious tanks aboard I-44

The concept of launching amphibious tanks from I-boats was promoted by Rear Admiral Kuroshima ("Gandhi") Kameto (44th), one of Admiral (Fleet Admiral, posthumously) Yamamoto Isoroku’s former staff operations officers and Pearl Harbor planner. Kuroshima was assisted by Cdr Fujimori Yasuo (former CO of I-121), a staff submarine operations officer.

Most submarine officers opposed the idea; nevertheless, Admiral Kuroshima persisted. The submarine captains were astonished and could not believe that the NGS was serious about the plan. One can but imagine the comments the submariners reserved among themselves for Admiral Kuroshima - an armchair Admiral who never held a command and had been in staff and training billets for the past 15 years.

On or about 4 May, they would depart the Japanese advance submarine base at Saipan. The submarines were to proceed to Majuro, arriving the evening before the attack. Submarine I-10 was to be tasked with periscope reconnaissance of the landing area prior to the attack. At least two other unidentified submarines would carry Naval Special Landing Force (SNLF) units to support the landing and demolish shore facilities. After the launch of the torpedoes the SNLF, supported by the tanks, had to overrun and demolish as many shore facilities as possible (including avgas tanks, barracks and storage depots) before the withdrawal to the submarines.

The approach flank of the main body was to be protected by submarines RO-104; RO-105, RO-106, RO-107, RO-108, RO-112 and RO-116. The submarine picket had a dual role – after the attack it was to intercept the carriers escaping from Majuro (similar to later Kaiten attacks against the anchorages).

The submarine carriers chosen to participate in the operation belonged to SubDiv 15. Their training HQ was located on the submarine tender TSUKUSHI MARU, then anchored at Kure. The training of the tank crews and their carriers commenced at Nasake Jima, a small island E of Kurahashi Jima and lasted ten days. There were no dedicated tank crews, so they were drafted from amongst midget submarine crews.

The first landing trials revealed many deficiencies of the Type 4 tanks, most of which were due to its underpowered engine. After the mother submarine surfaced, at least 20 minutes were necessary to remove the watertight engine covers and launch the tank with cold engine.

The submariners knew that the Americans had established a PT-boat base at Cape Torokina and that no IJN submarines had been able to get near the anchorage, much less surface for 20 minutes or more to launch tanks! They doubted whether the tanks' motors would even start, after being underwater for so long. The slow, noisy tanks would have to reach land safely, cross it, and enter the sea again without being detected.

On 26 March 1944, the I-36 departed Kure for a preliminary aerial recconnaissance of Majuro. This was conducted on 22 April and the I-36's E14Y1 "Glen" reported the presence of eleven carriers and three battleships.

On 26 April, Vice Admiral (Admiral, posthumously) Nagumo Chuichi, of Pearl Harbor and Midway, was personally responsible for the implementation of YU-GO. Nagumo expressed his opposition, deeming the scheme impossible to carry out, but on 1 May 1944, Admiral Toyoda Soemu took command of Combined Fleet. On 3 May, he stated the operation had to be carried out "whatever it costs”. The Naval General Staff shared Toyoda’s view because the successful implementation of the operation could become a "turning point of the whole war”.

In early May, C-in-C, Sixth Fleet, Vice Admiral (Admiral, posthumously) Takagi Takeo personally led the preparations for YU-GO from the sub tender TSUKUSHI MARU. He concentrated five submarines and 14 tanks at a small island off Katsurajima Channel in the Inland Sea. Several support vessels and a 30-ton floating crane also arrived from Kure to facilitate loading the tanks onto their carriers. Five submarines, each carrying one tank apiece, practiced crash-dives, full-speed underwater maneuvers and torpedo attacks in addition to launch exercises. Vice Admiral Takagi initially had been rather optimistic about the whole plan, but his disappointment grew rapidly. The death of Admiral Koga had a good deal to do with the plan slipping because there was a hiatus in command for nearly a month and the pace of events caught up with Admiral Toyoda.

ComSubDiv 15, Cdr Takahashi Chojuro, to convince the brass the plan would most certainly fail, suggested full-scale landing trials be carried out using a submarine carrying two tanks. The tests revealed the tanks were excessively noisy, extremely slow in the water and their tracks tended to slip if even the smallest obstacles were encountered. Also, it was found their engines were not entirely watertight and their covers tended to leak underwater, eventually flooding the tank's entire engine section.

In response to his superiors’ criticism, the designer of the Type 4, Cdr Hori Motoyoshi of the Kure Naval Yard Shipbuilding section pointed out he had been tasked with constructing a "pure” amphibian - not suited either for underwater transport or carrying torpedoes (each of which weighed a full ton).

On 12 May 1944, after the C-in-C, Sixth Fleet submitted a negative report to the NGS, YU-GO was postponed "for the time being”, i.e. until the deficiencies of the Type 4 tanks had been eliminated. The I-36 continued diving trials with two tanks in the Aki Nada until at least 23 May.

At the time set for the Japanese attack, Task Force 58 was en route from raids in the Southwest Pacific. It was spotted by several Japanese “snoopers” and had hit the islands of Truk (29-30 April), Satawan (30 April), and Ponape (1 May) as it headed to Majuro. The Japanese could execute the operation though Majuro was empty on 1 May on the assumption that the quarry would be there when the attack force arrived. In fact, Task Force 58 arrived on 3-4 May.

On 2 May, escort carriers FANSHAW BAY and KALININ BAY with their screen of four DEs on patrol in the Eastern Marshalls departed their patrol area. KALININ BAY headed for Majuro while the remaining ships went to Pearl Harbor. No Hunter Killer ASW group replaced these ships during the next two weeks and all ASW patrolling would be undertaken by shore based air and DE patrols.

On 3 May 1944, carriers WASP, ESSEX and SAN JACINTO with two AA cruisers and two destroyers left Pearl Harbor for Majuro. Battleship MASSACHUSETTS and carriers YORKTOWN, PRINCETON and MONTEREY together with seven destroyers departed Majuro for Pearl on the 6th, due to arrive on 11 May.

The U. S. Fifth Fleet's Intelligence Addendum to the Operations Order for Operation “Forager”, the Invasion of Saipan, Marianas, planned for mid-June noted that "the Japanese probably suspect but do not yet know that an amphibious operation is about to commence." The Addendum further noted that "it is believed that the Japanese Fleet is disposed of one division of new battleships, one division of cruisers and at least one squadron of destroyers at Halmahera (Moluccas) and three or four battleships, possibly three divisions of carriers, possibly two divisions of cruisers, about twenty destroyers and some auxiliary vessels at Tawitawi."

The intel report estimated "possibly two divisions of cruisers, possibly a battleship and some destroyers are operating NW of New Guinea”. It also noted "there are believed to be three replenishment forces, of which two are in the Philippines, and one enroute. Another replenishment force is possibly operating in the Surigao area awaiting rendezvous orders."

The report indicated that "in the Empire and the north there are three battleships, about seven heavy and light cruisers, two or three CVEs, plus destroyers and a number of submarines. Of the three carrier divisions operating in the Philippines, two had not been definitely located.”

The same intel report estimated that on 18 May 1944, the IJN's carrier and air strength in the Philippines consisted of three fleet carriers with 75 planes each, two others with 51 planes each and four light carriers with about 30 planes each. *3

The report also estimated shore-based air strength in Japan and the Western Pacific at more than 1,150 planes.*4

Except for the two “lost” IJN carrier divisions, the American Intel estimate was remarkably close. The lost" divisions may be have been nothing more than a failure to recover their latest call sign changes.

The relative strength of the IJN Mobile Force's and Task Force 58's ships and aircraft at or near Majuro during the intended Japanese attack probably would have been:*5

Key To Vessels:
Vessel Types Japanese American
Fleet Carriers x 5 (327 a/c) x 7 (658 a/c)
Light Carriers x 4 (121 a/c) x 8 (264 a/c)
Escort Carriers x 4 (100 a/c) x 3 (93 a/c)
x 3
x 2
x 7
Heavy Cruisers x 11 x 8
Light Cruisers x 2 x 9
Destroyers x 27 71
Destroyer Escorts x ? x 14
Submarines x 12 (10 tanks) x ?
Shore-based Aircraft x 272 x 417

Authors' Notes:
*1. IJN Mobile Force for YU-GO:
Carrier Division One: TAIHO, SHOKAKU, ZUIKAKU each with 27 VF, 27 VB, 3VS, 18 VT
Carrier Division Two: JUNYO, HIYO each with 27 VF, 18 VB, 6 VT and RYUHO with 27 VF, 6 VT
Carrier Division Three: CHITOSE, CHIYODA, ZUIHO each with 21 VF, 9 VT
Battleship Division One: YAMATO, MUSASHI, NAGATO
Battleship Division Three: KONGO, HARUNA
Cruiser Division Four: ATAGO, MAYA, CHOKAI, TAKAO
Cruiser Division Five: MYOKO, HAGURO
Destroyer Squadron Two: NOSHIRO and twelve destroyers
Destroyer Squadron Ten: YAHAGI and fifteen destroyers
Auxiliary Carrier Group: KAIYO, TAIYO, UNYO, SHINYO each with 25 VFB

*2. The principal characteristics of the tanks, known as Toku 4 Shiki Naika-Tei (Motor Boats, type 4 Special), were: Length: 11 meters, Beam: 4 meters, Draft : 2.25 meters, Displacement 19-22 tons, Speed :5 knots, Propulsion: one engine in a watertight pressure box, Armament: 2-13-mm machine guns; and 2-45-cm torpedoes with modified aircraft drop mechanisms. 18 of the craft were completed by mid-March. Their low speed was considered a draw back and they were never actually used in combat.

*3. IJN carrier divisions in the Philippines:
Cardiv 1: SHOKAKU, ZUIKAKU, TAIHO; 27 VF, 27 VB 18 VT, 3 VS each
Cardiv 2: HITAKA (HIYO), HAYATAKA (JUNYO); 27 VF, 18 VB, 6 VT each, RYUHO 27 VF, 6 VT
Cardiv 3: CHITOSE, CHIYODA, ZUIHO; 21 VF, 9 VT each

*4. IJNAF shore-based air:
Empire (combat units only): 250 VF, 120 VB, 80 VB(L), 55 VB (M), 10 VFB, 35 VF(p)
Marianas: 186 VF, 48 VB, 82 VB(L), 15 VB(M), 6 VFB, 16 VF(p)
Carolines: 42 VF, 12 VB, 6 VB(M), 6VF(p)
Palau and Yap: 52 VF, 24 VB, 9 VB(M), 2 VFB, 11 VF(p)
Philippines, Dutch East Indies and New Guinea; 70 VF, 16 VB, 61 VB(M), 2 VFB, 68 VF(p)

*5. USN Fifth Fleet at Majuro:
CVEs: LONG ISLAND, KALININ BAY, ALTAMAHA (each with 22 VF, 9 VT approx)
DDs: 64, DEs 14

USN units en route to Majuro:

USN patrol squadrons in the Marshall Islands vicinity:
Midway: 6 VP; Johnston: 3 VP; Tarawa: 3 VP; Majuro: 15 VP; Abemama: 1 VP; Makin: 1 VP; Roi: 1 VP, Eniwetok: 2 VP, 36 VB(H); Kwajalein: 9 VP; Ebeye: 5 VP. These aircraft conducted routine radial patrols out to 700 miles that could be extended to 1200 if the situation dictated.

Other USN shore-based air:
Majuro: 97 VF, 55 VB, 27 VT, 3 VB(M)

Eniwetok: 46 VF, 33 VB, 36 VB(H), 10 VT

Combat aircraft only shown.

Thanks go to Anthony Tully and also to readers John Hay and Christian Kellum for picking up some errors.

-David Dickson, Bob Hackett and Sander Kingsepp


English Language:
  • In an article in NEWSWEEK on 10 March 10, 1944, Admiral William V. Pratt, USN (ret.) former CNO wrote, “and we shouldn’t forget that at Pearl Harbor the Japs (sic) caught us by surprise once before. They will try their damnedest to do it again.”
  • The account of the meeting on MUSASHI is from correspondence between David Dickson and Captain Yamamoto in the 1970s.
  • The table of organization is based on Morison Vol. VIII and correspondence with the Japanese National Defense Force History Section. The air groups are standard not on board counts for the period.
  • The Base Air Force figures are from Japanese Monographs 90 and 91 and Morison Vol. VIII.
  • The submarines earmarked for the operation are based on correspondence with the JNDF History Section.
  • The particulars of the Type 4 tanks are from WARSHIPS OF THE IMPERIAL JAPANESE NAVY, 1869-1945 by Hansgeorg Jentschura, Dieter Jung and Peter Mickel, USNIP 1977 (p. 272), THE JAPANESE NAVY AT THE END OF WW2, by Shizuo Fukui, We, Inc. (illustration 203). There is a photograph of one of these machines in Maru NIHON GUNKAN at p. 177.
  • American ship movements and aircraft availability are from Monthly Ship Movement Summaries and Aircraft Availability summaries for the period in question.
  • Boyd, Carl and Akihiko Yosida. "The Japanese Submarine Force and World War II." Naval Institute Press, 1995.

    Japanese-Language Sources:
  • Fuchida Mitsuo, Okumiya Masatake. Kidō Butai.
  • Itakura Hideo. Itakura-kanchō Sensuikan Senki (The Memoirs of the Submarine Skipper Itakura).
  • Takenouchi Akira. Nihon no Sensha (Japanese Tanks)
  • Torisu Kennosuke. Ningen Gyorai, Kaiten to Wakabito-tachi (Human Torpedoes, Kaitens And Young Pilots).

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