'Hailstone' was a US air, surface ship and submarine attack on Japanese shipping in Truk atoll of the Caroline islands group in concert with 'Galvanic' (17/18 February 1944).

Truk is a collection of hilly islands, the tips of drowned mountain peaks, surrounded by a large barrier reef 40 miles (65 km) across with five entrance passes. In the Pacific War of World War II, Truk possessed the best fleet anchorage in the Japanese mandated territories (ex-German Caroline, Marshall and Mariana island groups), and it was believed that before World War II that Japan had turned Truk into a 'Gibraltar of the Pacific' despite the anti-fortification strictures of the Washington Naval Treaty if 1922. It was known from intelligence garnered from radio intercepts that the 4^th Fleet had its headquarters in the group and, located as it is in the centre of the Caroline islands group, Truk was ideally situated as a nexus of the Japanese empire’s interior lines of communications.

The Japanese had in fact developed considerable base facilities here before the war, including four separate airfields and storage for 77,200 tons of fuel oil, including a 10,000-ton underground tank and two 33,600-ton above-ground steel tanks: this was the largest Japanese fuel depot outside the home islands. Truk was also an important submarine base.

However, Truk was never as strong as the Americans initially believed, and it lacked both piers and shore services, forcing ships to anchor in the lagoon and have supplies delivered by lighter, and to run their machinery on a constant basis for the provision of electrical power and fresh water. Some of these deficiencies were remedied as the war progressed.

Japanese merchants had first visited Truk in 1891, and the island group was seized by the Japanese from the Germans during World War I. By the time war broke out in December 1941, there were 3,000 Japanese civilians and 18,000 natives in the group. The latter were excluded from the islands taken over by the Japanese for exclusively military use.

The principal town was Dublon, on the island of the same name. Dublon had about 1,200 buildings and was also the home for the facilities, including a 2,500-ton floating dry dock, required to make temporary repairs to warships. The construction of fortifications did not begin until 1940 and was not pressed until January 1944. The garrison reached a maximum of 7,500 army and about 4,000 navy troops by February 1944, and coastal guns were sited to cover all five of the lagoon’s entrances, which were also protected with electrically controlled mines. However, there were only 40 anti-aircraft guns without fire-control radar.

The facilities completed by February 1944 were a 3,345-ft (1020-m) bomber airstrip, a combined seaplane base and fighter airstrip, coast-defence and anti-aircraft guns, radar, and a torpedo boat base and torpedo storage on Moen; main docks, a seaplane base, a submarine base, a 2,500-ton floating dry dock, oil, torpedo, and munitions storage, coast-defence and anti-aircraft guns, and an aviation repair and supply station on Dublon; a supply centre with a pier, search radar and two 127-mm (5-inch) dual-purpose guns on Fefan; search radar and a torpedo boat base on Uman; an airstrip 33,345 ft (1020 m) long and 260 ft (80 m) wide, and revetments on Eten; an airstrip 3,905 ft (1190 m) long and 330 ft (100 m) wide, eight 127-mm (5-in) guns, four 80-mm (3.15-in) dual-purpose guns and three medium anti-aircraft guns on Param, a radio direction finding station on Ulalu; three 203-mm (8-in) guns on Udot; and four 152-mm (6-in) coast-defence guns, a battery of anti-aircraft guns, radar and a torpedo boat base on Tol.

Truk atoll was the only major Japanese air base within range of the Marshall islands group, and was a significant source of support for Japanese garrisons located on islands and atolls throughout the central and south Pacific. The base was the key logistical and operational hub supporting Japan’s perimeter defences in the central and south Pacific.

On the eve of 'Hailstone', the 4^th Fleet was commanded by Vice Admiral Masami Kobayashi, replaced on the day after the US operation by Vice Admiral Chuichi Nagumo.

'Hailstone' was undertaken as a precursor to 'Catchpole', the seizure of Eniwetok atoll in the Marshall islands group as the Americans expected that the assault on this strategic atoll would prompt an air assault by Japanese long-range aircraft based around Rabaul in New Britain and Kavieng in New Ireland, and a combined naval and air assault from the Japanese navy’s main central Pacific base at Truk. The latter had in fact been relegated to a secondary position as recently as 10 February by the Japanese decision to relocate the carriers, battleships and heavy cruisers of Admiral Mineichi Koga’s Combined Fleet to the Palau islands group. However, it was via Truk that most of the aircraft, troops, supplies and matériel were staged into the Solomon islands group, Bismarck islands group, New Guinea, New Britain and New Ireland for the support of the Japanese conquest of this South-West Pacific area and, later, to defend the perimeter of the expanded Japanese empire.

In February 1942 the headquarters of Admiral Shigeyoshi Inoue’s 4^th Fleet was relocated from the Palau islands to Dublon island. In July Admiral Isoroku Yamamoto’s Combined Fleet (elements of Vice Admiral Mizumi Shimizu’s 1^st Fleet, Admiral Nobutake Kondo’s 2^nd Fleet and Vice Admiral Chuichi Nagumo’s 3^rd Fleet, together with Vice Admiral Marquis Teruhisa Komatsu’s 6^th Submarine Fleet) arrived at Truk. At its height of operation up to 1,000 ships were sometimes located in the lagoon.

While it was a superior anchorage and well defended, Truk’s repair facilities were limited. Truk was defended not only by major naval surface and air forces, but also by significant ground forces. These latter included Lieutenant General Shunsaburo Mugikura’s 52^nd Division (69^th and 150^th Regiments but not the 107^th Regiment, which had been detached to Ponape), most of the 51^st Independent Mixed Brigade, Japanese army service and support troops, and the Japanese navy’s 4^th Base Force, 41^st Guard Force, 101^st Sasebo Special Naval Landing Force, and significant air base service units as well as construction units with 5,200 labourers. Mugikura led both the 52^nd Division and the 31^st Army, and also commanded the Truk District Group.

In May 1945 there were an estimated 13,600 Japanese army troops and 10,600 Japanese naval personnel on the islands.

Several of the atolls around Truk were defended, and these included Woleai (5,500 men of the 50^th Independent Mixed Brigade and 44^th Guard Force), Pulawat (3,500 men of the 11^th Independent Mixed Brigade and a naval detachment), Nomoi (2,400 men of part of the 51^st Independent Mixed Brigade and a naval detachment), Ponape (8,000 men of the 52^nd Independent Mixed Brigade, 107^th Regiment and 42^nd Guard Force), and Kapingamarang (400 men of army and naval detachments).

The US forces had no desire or indeed need for a direct assault on Truk, which would instead be neutralised through its isolation by air and sea in a manner already accomplished so effectively against Rabaul. However, Truk atoll was the only major Japanese air base within range of the Marshall islands group and was a significant source of support for the Japanese garrisons located on islands and atolls throughout the central and southern parts of the Pacific Ocean. To ensure air and naval superiority for 'Catchpole', therefore, Vice Admiral Raymond A. Spruance, commanding the Central Pacific Force that became the 5^th Fleet in April 1944, ordered a two-day assault on Truk.

The Central Pacific Force had five fleet and four light carriers (with a total of 589 aircraft), battleships Alabama, Iowa, Massachusetts, New Jersey, South Dakota and Washington, 10 cruisers (including the heavy cruisers Baltimore, Minneapolis and New Orleans), and 27 destroyers.

On 12/13 February three task groups of Vice Admiral Marc A. Mitscher’s Task Force 58 1 departed Majuro atoll and refuelled at sea from five oilers on 15 February in final preparation for 'Hailstone'.

Under Spruance’s command, moreover, was TG50.9 comprising the battleships Iowa and New Jersey, heavy cruisers Minneapolis and New Orleans, light carrier Cowpens (see above), and destroyers Izard, Charrette, Burns and Bradford.

'Hailstone' began early on 17 February with an attack on the 365 Japanese aircraft at Truk using fighters from the five fleet carriers, while the aircraft of the light carriers operated against shipping. Toward the fall of night on 17 February the Japanese counterattacked with seven Nakajima B5N 'Kate' single-engined torpedo-bombers. These achieved one torpedo hit on Intrepid, which returned to Majuro with Cabot, two cruisers and four destroyers. Another Japanese air attack slightly damaged the battleship Iowa with a single bomb hit.

On 18 February the aircraft of the remaining fleet carriers attacked shipping targets: 1,250 sorties were flown and 400 tons of bombs were dropped. As noted above, however, the heavy units of the Combined Fleet had recently transferred to Singapore, so only a small number of warships and numerous auxiliaries were sunk in the lagoon, which never again regained its importance as a Japanese base.

In Truk the submarine tenders Rio de Janeiro Maru and Heian Maru, as well as six tankers and 17 freighters totalling 137,019 tons were sunk, and about 250 aircraft were destroyed. TG50.9 steamed round Truk to intercept fleeing ships.

On 17 February the training cruiser Katori and the destroyer Maikaze were sunk to the north-west of Truk by the gunfire of the cruisers Minneapolis and New Orleans, supported by the destroyers Radford and Burns, and the last also sank the submarine chaser Ch-24; the light cruiser Naka was sunk to the west of Truk by aircraft from the carriers Bunker Hill and Cowpens; the destroyer Fumitsuki and submarine chaser Ch-20 were sunk to the south-west of Truk on 18 February by aircraft from the carrier Enterprise; the destroyers Oite and Tachikaze were sunk by carrierborne aircraft to the west of Truk; and the armed merchant cruisers Aikoku Maru and Kiyosumi Maru were sunk to the north-west of Truk by carrierborne aircraft.

The destroyers Shigure and Matsukaze, submarines I-10 and Ro-37, target ship Hakachi, ammunition ship Soya and seaplane tender Akitsushima were damaged.

Only the destroyer Nowake managed to escapes the salvoes of the covering battleships, fired from a distance of 39,000 yards (35660 m).

During the operation the US submarine Seal was stationed near Ponape, and Searaven and Darter patrolled near Truk as planeguards to rescue airmen forced down onto the water. The exits from Truk were covered by submarines.

Skate sank the light cruiser Agano on 16 February (Oite rescued 523 of the cruiser’s survivors and returned to Truk lagoon to assist in its defence with her anti-aircraft guns, but soon fell victim to air attack with Agano's survivors still on board, killing all of them and all but 20 of Oite's crew); Tang sank four ships (14,300 tons) and escaped from a depth-charge attack by the frigate Amakusa and submarine chasers Ch-31 and Cha-24; Sunfish sank two ships (9,437 tons); Aspro on 15 February sank the submarine I-43 as the latter was transporting marines of the Sasebo Special Naval Landing Force; Burrfish and Dace sank no ships; and Gato sank three ships (12,903 tons) and three small craft.

On 20 February aircraft from TG58.1 attacked Jaluit and on 23 February TG58.3 bombarded Tinian and Rota in the Mariana islands group, and TG58.2 bombarded Saipan and Tinian in the same island group.

Japanese attempts to attack on 22 February failed. Altogether the Japanese lost 21,300 tons of shipping to attacks by carrierborne aircraft. In total, therefore, 'Hailstone' sank the Japanese light cruisers Agano, Katori and Naka, destroyers Oite, Fumizuki, Hagio, Isogu, Maikaze and Tachikaze, auxiliary cruisers Akagi Maru, Aikoku Maru and Kiyosumi Maru, submarine tenders Heian Maru and Rio de Janeiro Maru), three other smaller warships (including submarine chasers Ch-24 and Shonan Maru 15), aircraft transport Fujikawa Maru, and 32 merchant ships totalling some 200,000 tons.

Some of the ships were destroyed in the anchorage and some in the area surrounding Truk lagoon. Many of the merchant ships were loaded with reinforcements and supplies for Japanese garrisons in the central Pacific area. Very few of the troops aboard the sunken ships survived, and little of their cargos was recovered. Some 275 Japanese aircraft were destroyed, most of them on the ground. Many of the aircraft were in various states of assembly, having just arrived from Japan in disassembled form aboard cargo ships, and only a few of the assembled aircraft were able to take off in response to the US attacks. About 30 of the Japanese aircraft which did take-off were destroyed by US carrierborne fighters or the gunners of the US dive-bombers and torpedo aircraft.

The US force lost 25 aircraft, most of them to the intense anti-aircraft fire from Truk’s defences. About 16 aircrew were rescued by submarine or amphibious aircraft.

In overall terms, 'Hailstone' effectively ended Truk’s role as a major threat to Allied operations in the central Pacific. The attack also denied the Japanese garrison of Eniwetok any real hope of reinforcement and support during the invasion that began on 18 February, greatly assisting US forces in their reduction of that island.

The Japanese later relocated about 100 of their remaining aircraft from Rabaul to Truk, but these aircraft were attacked by US carrier forces in another attack on 29/30 April 1944, which destroyed most of them when 92 bombs were dropped over a 29-minute period. The April 1944 attacks found no shipping in Truk lagoon, and were the last major attacks on Truk during the war.

So far as the Japanese navy was concerned, a longer-term effect of 'Hailstone' was the politically motivated replacement of Admiral Osami Nagano, the chief of navy operations, by the politically pliant Admiral Shigetaro Shimada. Truk was thus isolated and bypassed by the Allied forces as they continued their advance toward Japan by invading other Pacific islands such as Guam, Saipan, Palau, and Iwo Jima. Cut off, the Japanese forces on Truk and other central Pacific islands ran short of food and faced starvation before Japan surrendered in August 1945.

The islands were often attacked by the Consolidated B-24 Liberator and Boeing B-29 Superfortress heavy bombers of newly arrived bomber units, which used the attacks to hone their skills before being tasked with more dangerous operations. On 2 September 1945 Mugikura and Hara surrendered Truk and its outlying islands (130,000 military and civilian personnel) to Vice Admiral George D. Murray, who had arrived in the heavy cruiser Portland to accept this largest Japanese surrender of the Pacific Ocean Areas.