Operation Tan 2

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'Tan 2' was a Japanese kamikaze air raid on the US naval base and anchorage in Ulithi atoll (11 March 1945).

In September 1944, after the US capture of the Mariana islands group in 'Forager' and Peleliu island of the Palau islands group in 'Stalemate II', Ulithi atoll in the Caroline islands group became a primary forward anchorage for major elements of the US Pacific Fleet. A huge atoll made up of 40 islets in the western part of the Caroline islands group, some 350 miles (565 km) to the south-west of Guam in the Mariana islands group, Ulithi was the centre of preparation for the USA’s next advances though the Pacific Ocean, namely the 'Detachment' and 'Iceberg' operations for the descents on and conquests of the islands of Iwo Jima and Okinawa respectively.

The anchorage at Ulithi could accommodate 700 large ships, a capacity greater than that of either Majuro atoll or Pearl Harbor. The headquarters of Admiral Soemu Toyoda’s Combined Fleet therefore started to develop the plan for a major assault on this immensely tempting target. Commander Ryosuke Nomura concentrated on a scaled-down and more practical expression of the concepts embodied in earlier Japanese thinking about attacks on US Navy fleet anchorages, and included the use of kamikaze aircraft. Whereas the early plans relied on technical innovations such as torpedo-firing amphibious tanks and several intermediate landings for aircraft covering huge distances, Nomura’s scheme stretched the limits of existing technology, and was based on a flight of almost 10 hours to Ulithi atoll from Kyushu, the south-western island of the Japanese home islands.

Just as they had for 'Yu', the Japanese believed that the success of the operation would paralyse the US Navy and become the long-anticipated 'turning point of the war'. Vice Admiral Matome Ugaki, commanding the Japanese navy’s 5th Air Fleet, endorsed 'Tan 2', and the plan was initially scheduled for execution at a time late in February 1945, but was then delayed and the required Azusa Tokubetsu Butai (Azusa Special Attack Unit) was established only on 7 March.

The attack was planned on the basis of a force of 24 Yokosuka P1Y Ginga 'Frances' two-engined bombers, originally of the 762nd Naval Air Group. The three-seat P1Y could carry two 1,102-lb (500-kg) bombs or one 1,764-lb (800-kg) torpedo. Five Kawanishi H8K2 'Emily' four-engined flying boats of the 801st Naval Air Group and four land-based bombers were to provide pathfinder and weather reconnaissance support.

Early in February 1945, the 141st Naval Air Group based at Kisarazu on Tokyo Bay was ordered to dispatch a detachment of seven C6N aircraft to Truk. On 11 February, these aircraft departed, two soon returning and one ditching en route. After an overnight stop at Chidori airfield on Iwo Jima, the four surviving C6N machines landed at Truk, but one was wrecked on landing. Designed for carrierborne service, the C6N required specialised maintenance, but no fuel or spare parts were available at Truk.

Vice Admiral Shigeyoshi Miwa’s 6th Fleet deployed three submarines to support the attack. The 2,607-ton I-58 fleet boat was to serve as a radio beacon for the P1Y force; the small 370-ton cargo submarine Ha-106 was to cruise off Minami Daito Jima to pick up ditched aircrews; and the 1,779-ton I-366 transport submarine was to make a trip to Meleyon islet in the Woleai atoll via Truk with 60 tons of cargo including 33 tons of aviation fuel.

I-366's voyage was plagued by technical accidents from the start, and the boat soon signalled that it was returning to base. Repairs began immediately and eight hours later the submarine again departed. I-366 spent the night off Tateyama at the southern tip of the Boso peninsula, and at 03.00 on the next morning raised anchor and headed to the south. A few days later, I-366 encountered a low-pressure area with strong winds and an increasingly rough sea: while travelling on the surface, the boat struggled against 30-ft (9.1-m) waves which repeatedly flooded her Diesel engines and carried away a significant part of her deck cargo. By 10 February the weather had improved. At 04.48, a look-out reported a cruiser on a south-westerly bearing. I-366 had neither torpedo tubes nor any other means to attack. When two more vessels appeared on the horizon to the south, the boat was dived to 200 ft (60 m) to continue the mission. Early in the morning, I-366 surfaced off Moen island at Truk and contacted the submarine base, but was advised not to enter the anchorage because of US air activity. I-366 submerged and surfaced once again at 12.30. Her cargo was unloaded but the operation took some time. I-366 departed the anchorage after the fall of night and headed for Meleyon.

The ground crews at Truk made good use of the materials delivered by I-366. Early in the morning of 13 February, the only flyable C6N took off for a reconnaissance sortie over Ulithi. Flying at 42,650 ft (13000 m) as the aeroplane neared the anchorage, the observer decided not to use the long focal length camera installed in the rear fuselage as there were no carriers at Ulithi. He did take a few pictures with his hand-held camera, and photo interpreters at Truk later found the entrance to Ulithi’s anchorage was protected with anti-submarine nets. After landing back at Truk, the pilot reported the results of the flight directly to Vice Admiral Chuichi Hara, commanding the 4th Fleet, who passed on the information to the headquarters of the Combined Fleet in the Japanese home islands. Here Toyoda decided to postpone the operation until a time early in March.

On 5 March a C6N photographed 16 carriers in the Ulithi anchorage, and four days later a C6N located and photographed six fleet carriers and nine escort carriers of Task Force 58 at Ulithi, and spotted a group including four carriers entering the atoll from the north-east. This information was reported by radio as the C6N headed back to base, escaping from three Lockheed P-38 Lightning fighters somewhere between Ulithi and Truk. After the C6N had landed at Truk, Japanese photo interpreters identified the carriers as one 'Saratoga' type, four 'Essex' type, three 'Independence' type and seven escort carriers, as well as four unidentified ships.

When he received this information, Toyoda ordered that 'Tan 2' should be implemented on 11 March. On the same day, the submarine I-58, carrying four kaiten suicide attack submarines, was en route to Iwo Jima to attack Spruance’s 450-ship invasion fleet, but was then recalled to Okino-torishima to participate in 'Tan 2'. The boat’s captain had expected to arrive at the kaiten boats' launch position off Iwo Jima within 90 minutes, and now jettisoned two of the four boats as I-58 proceeded toward Okino-torishima on the surface at maximum speed, reaching the reef as ordered at sunset on 10 March.

On 11 March Ugaki addressed the Azusa Special Attack Unit before its 24 aircraft took off from Kanoya on their one-way mission at 09.00 under the command of Lieutenant Naoto Kuromaru. But the departure of the H8K weather reconnaissance flying boats from Kagoshima had been delayed. Their machines finally got airborne just two hours before the Azusa Special Attack Unit took off. The flying boats were divided into two units: the first comprised three 'boats, including the weather reconnaissance flying boat, and the second comprised two 'boats. The H8K 'boats were to guide the P1Y aircraft to Minami-Daito Jima and then return.

About four hours into the mission, the formation of P1Y aircraft, each carrying one 1,764-lb (800-kg) bomb, passed the submarine Ha-106 off Minami-Daito Jima. Five P1Y machines turned back because of engine troubles and later landed on Minami-Daito Jima, and three others crashed. It is uncertain whether or not Ha-106 rescued any men of the air crews. The boat returned to Kanoya on the same day and then returned to its station in the Minami-Daito Jima area for two more days.

The submarine I-58 took up station off Okino-torishima and functioned as a radio beacon for the bombers. Engine troubles and other problems continued to afflict 'Tan 2'. One of the H8K2 pathfinder 'boats disappeared and six P1Y machines had to turn back to Kanoya. Rain squalls developed in the vicinity of Okino-torishima and forced the attack force, already behind schedule, to climb above the clouds, thereby losing visual checkpoints. The weather also forced the attack aircraft to detour around many rain squalls. These detours increased the distance, already close to the P1Y’s maximum, and burned precious fuel. Other aircraft developed engine trouble and landed on islands along the way, and two ditched at sea.

Eight hours after take off the surviving aircraft of the Azusa Special Attack Unit descended through the overcast. As a result of a navigational error and unexpected head winds, they found themselves near Yap island, 120 miles (195 km) to the west of Ulithi. After seeing the P1Y machines over Yap, one of the H8K2 'boats went to Meleyon, and four P1Y machines crash-landed on Yap.

The sun set at 18.52, and only two of the original 24 P1Y bombers reached Ulithi, both well after dark. The bombers approached Ulithi at high altitude, dropped chaff to deceive the US radars, then dived down to attack at wave-top height. The two bombers flew into Ulithi’s anchorage undetected, and as a result no alert had been sounded. The islets and ships were all well lit, and the ships' crews relaxed.

At 20.07, a P1Y hit the starboard side of the fleet carrier Randolph, which was anchored off Sorlen islet. The bomber impacted aft just below the flight deck, but had so little fuel left in its tanks that it did not burst into flames, though the detonation of its bomb destroyed aircraft on the flight and hangar decks. The carrier was badly damaged, 26 men being killed and another 105 wounded.

The other P1Y mistook Sorlen islet for another aircraft carrier and ploughed into it.

About 23.30, an H8K2 pathfinder 'boat landed at Meleyon, but there was no fuel on the island so the 'boat was sunk by machine gun fire during the next day. The garrison of Meleyon consisted of the 50th Independent Mixed Brigade and 44th Guard Unit, all of whose men were on the verge of starvation. They waited for a submarine for 57 days and survived by eating rats. Two months later, on 10 May, the transport submarine I-369 arrived at Meleyon and rescued the survivors.

The Combined Fleet's voice communications service intercepted several plain language radio messages about an air raid on Ulithi. Based on these intercepts, Toyoda was optimistic about the results of 'Tan 2', but on the next day a C6N from Truk discovered that all the carriers were still there and no fuel was visible in the water. Only then was it clear the attack had failed.

Randolph was repaired locally and returned to action in early April 1945, serving as the flagship of Task Force 58 during the latter part of the Okinawa campaign.