Operation Hailstone


Operation Hailstone was a large-scale United States Navy air and surface attack on Truk Lagoon on 17–18 February 1944, conducted as part of the American offensive drive against the Imperial Japanese Navy in the Pacific Ocean theatre of World War II.

Background

Japanese troops occupied Micronesia, including the Caroline Islands, in 1914 and established Truk as a base as early as 1939. The lagoon was first built up as an anchorage for the Imperial Japanese Navy's 4th Fleet, its "South Seas Force". After the outbreak of war with the United States, the 4th Fleet was put under the command of the Combined Fleet, which continued to use Truk as a forward operating base into 1944. In addition to anchorages for warships and port facilities for shipping between the home islands and the Southern Resources Area, five airfields and a seaplane base were constructed at Truk, making it the only major Japanese airfield within flying range of the Marshall Islands. As Imperial Japan's main base in the South Pacific theatre of World War II, it was often described as the "Japanese Pearl Harbor".
There was a myth that Truk was heavily fortified, and it was given nicknames like "the Gibraltar of the Pacific." Despite the impressions of U.S. Navy leaders and the American public concerning Truk's projected strength as the "Japanese Pearl Harbor", Truk was never significantly reinforced or protected against land attack. In fact,
Japanese development of Truk's fortifications instead began in earnest in late 1943, with defensive measures being taken against a potential U.S. invasion. Airfields were extended and shore batteries were erected. Nonetheless, Japanese preparations were inadequate with few anti-aircraft guns and inadequate radar warning. The continued separate command arrangements made it hard for Navy fighter planes and Army troops to coordinate defensive efforts.

Prelude

Because aircraft stationed at Truk could potentially interfere with the upcoming invasion of Eniwetok, and because Truk had recently served as a ferry point for the resupply of aircraft to Rabaul, Admiral Raymond Spruance ordered Vice Admiral Marc Mitscher's Fast Carrier Task Force, designated TF 58, to carry out air raids against Truk. Three of TF 58's four carrier task groups were committed to the operation. Their total strength consisted of five fleet carriers and four light carriers, carrying more than 500 warplanes. Supporting these aircraft carriers was a task force of seven battleships and numerous heavy cruisers, light cruisers, destroyers, and submarines.
The Japanese, meanwhile, understood the weakness of their position at Truk. The IJN had begun withdrawing major units from the anchorage as early as October 1943. The effective abandonment of Truk as a forward operating base accelerated during the first week of February 1944, following Japanese sightings of U.S. Marine Corps PB4Y-1 Liberator reconnaissance planes sent to reconnoiter the area.

Battle

Air attacks

The three carrier task groups committed to Hailstone moved into position and began launching their first fighter sweep 90 minutes before daybreak on 17 February 1944. No Japanese air patrol was active at the time, as the IJN's 22nd and 26th Air Flotillas were enjoying shore leave after weeks on high alert following the Liberator sightings. Similarly problematic for the Japanese, radar on Truk was not capable of detecting low-flying planes—a weakness probably known and exploited by Allied intelligence organizations. Furthermore, radar stations were not adequately manned and telephone communications were poor. Because of these factors, U.S. carrier aircraft achieved total surprise.
Japanese pilots scrambled into their cockpits just minutes before TF 58 planes arrived over Eten, Param, Moen, and Dublon Islands. Though more than 300 Imperial Japanese Navy Air Service and Imperial Japanese Army Air Service planes were present at Truk on the first day of attacks, only about half of them were operational, compared with over 500 operational aircraft among the carriers of TF 58. U.S. Navy fighter pilots in their Grumman F6F Hellcats, with the advantages of speed, altitude, armor, and surprise, achieved a one-sided victory against IJNAS pilots flying Mitsubishi A6M Zero fighters. As many as 30 of the 80 Zeros sent up in response to the fighter sweep were shot down during the battle. Only token aerial resistance was encountered for the rest of the morning; almost no Japanese aircraft were present by the afternoon.
Due to the lack of air cover or warning, many merchant ships were caught at anchor, with only the islands' anti-aircraft guns for defense against the U.S. carrier planes. Some vessels outside the lagoon already steaming towards Japan were attacked by U.S. submarines and sunk before they could make their escape. Other Japanese ships attempting to flee via the atoll's North Pass were bottled up by aerial attack and by Admiral Spruance's surface force, Task Group 50.9, which circumnavigated Truk, bombarding shore positions and engaging enemy ships.
Torpedo bomber and dive bomber squadrons from the American carrier air groups were responsible for the bulk of the damage inflicted on Japanese ground facilities. Early on the first day of Hailstone, Grumman TBF Avenger torpedo bomber squadrons from Enterprises Carrier Air Group 10 and Intrepids CAG-6 dropped fragmentation and incendiary bombs on runways at Eten Island and the seaplane base on Moen Island. Dozens of aircraft were damaged or destroyed on the ground, further blunting any possible response by the Japanese to the strikes. Subsequent joint attacks by dive bombers and Avenger torpedo bombers cratered runways and destroyed hangar facilities.
Morning strikes were also launched against shipping targets in the lagoon. Lieutenant Commander James D. Ramage, commanding officer of Dive Bombing Squadron 10, is credited with sinking the previously damaged merchant tanker Hoyo Maru. Lieutenant James E. Bridges and his crew in one of Intrepids Torpedo Squadron 6 Avengers scored a direct hit on the ammunition ship. The torpedo detonation set off the ammunition carried near the bow resulting in a tremendous explosion which immediately sank the ship taking down 945 crew and passengers, and also engulfed Bridge's plane too.
By the second and third strikes of the day, CAG action reports listed the apparent enemy mission as "escape". Japanese ships that were able to make for open sea steamed for the North Pass exit from the lagoon while weathering repeated aerial attacks. One particular group of warships – cruiser Katori, auxiliary cruiser Akagi Maru, destroyers Maikaze and Nowaki, and minesweeper Shonan Maru – was given special attention by carrier bombers. Multiple air groups attacked these ships, inflicting serious damage. Yorktowns dive- and torpedo-bombing squadrons claimed two hits on the violently maneuvering Katori and hits on another cruiser and multiple destroyers; Essex bombers claimed five hits on a, stating that the ship was stopped dead in the water after the attack. Akagi Maru was sunk by air attacks.

Surface action

At this point, reports reached Admiral Spruance concerning the group of warships fleeing through North Pass. Spruance was so adamant on engaging in ship-to-ship combat that his carrier commander, Admiral Mitscher, ordered his CAGs to stop attacking Katori and her companions. Spruance put himself in tactical command of Task Group 50.9, made up of four destroyers, heavy cruisers Minneapolis and New Orleans, and the new battleships and, which he personally led in a surface engagement against the previously damaged Japanese ships. The battered Japanese ships did not stand much of a chance against Task Group 50.9, though members of his staff saw Spruance's decision to engage in surface action when aircraft likely could have achieved similar results as needlessly reckless. Indeed, the Japanese destroyer Maikaze managed to fire torpedoes at the battleship New Jersey during the engagement. Fortunately for Spruance, the torpedoes missed, and the "battle" ended with predictably one-sided results. The U.S. Navy surface combatants received virtually no damage, and it was the only time in their careers that Iowa and New Jersey had fired their main armament at enemy ships.
Meanwhile, New Jersey's 5-inch guns combined fire with U.S. cruisers to sink Maikaze and Shonan Maru, while Iowa targeted and sank Katori, which was already dead in the water, with numerous hits from her main battery. Nowaki was the only Japanese ship from this group to escape, sailing through a gauntlet of fire from Iowa and New Jersey, only suffering very minor damage at the hand of a straddle from a high capacity, 16-inch round from New Jersey at the extremely long range of 35,700 yards.

Japanese night raid

Retaliation for the day's strikes arrived late at night in the form of small groups of Japanese bombers flying out of Truk and Formosa probing the task groups' defenses. From roughly 21:00 on 17 February to just minutes past midnight on 18 February, at least five groups of between one and three enemy planes attempted to sneak past screening ships to strike at the fleet carriers. One lone Rikko-type twin engine bomber from the 755th Kōkūtai (Genzan Air Group) made its attack on Task Group 58.2 and torpedoed the starboard quarter of the Intrepid, damaging steering control and killing 11 sailors. Intrepid was forced to retire to the U.S. for repairs and did not return to combat until August 1944.

Assessment of attack

The attacks on Truk have often been described as the Americans' payback for the Japanese attack on Pearl Harbor, although American carrier aircrews were disappointed that major Japanese surface warships were absent, as they had been withdrawn prior to the raid. The only warships that were left to the Americans to attack were relatively insignificant, consisting of two light cruisers and four destroyers plus some auxillaries.
Nonetheless, Japanese losses at Truk were severe as these affected the navy's logistical capabilities. Truk had been a key center for aircraft distribution and repair in supporting Japanese naval aviation. Some 17,000 tons of stored fuel were destroyed by American airstrikes. Shipping losses totaled almost 200,000 tons, including several precious fleet oilers, of which the Japanese navy had a dwindling number by early 1944. Vessels sunk at Truk represented almost one-tenth of total Japanese shipping losses between 1 November 1943 and 30 June 1944. At this point in the war, Japan's industrial capability was unable to replenish such losses in ships and aircraft, in contrast to the American rearmament after the Pearl Harbor raid. In contrast to Truk, Pearl Harbor's oil storage tanks and repair yards had remained untouched.
After the Truk attacks, the Imperial Japanese Navy General Staff blamed Masami Kobayashi and relieved him of his command two days later. On 30 May 1944, Kobayashi was forced from active service and on 31 May 1944, he went into the reserves.

Aftermath until war's end

After Operation Hailstone, the U.S. deployed aircraft and submarine to isolate Truk, which began the effective severance of Japanese shipping lanes between the home islands and critical fuel supplies to the south. The ultimate effect of this interdiction was evident during the Battle of Leyte Gulf in October 1944, when IJN forces were compelled to sortie separately from Japan and Lingga Roads due to fuel constraints. The neutralization of Truk and the seizure of Eniwetok paved the way for the upcoming invasion of Saipan, which placed U.S. land-based heavy bombers within range of the Japanese home islands for the first time in the war.
No significant Japanese naval buildup occurred at Truk after Operation Hailstone. Subsequently, the garrison made limited attempts to rebuild Truk as a bomber air base and to increase its anti-aircraft defenses. In February, the Japanese command decided to pull all remaining Japanese airmen and their crews from Rabaul, which was earlier cut off by the Allies' island hopping campaign. Between 70 and 120 Japanese aircraft flew from Rabaul to Truk on the morning of 19 February, right after Operation Hailstone. Aircraft mechanics attempted to leave Rabaul on 21 February but their ship, the Kokai Maru (1939)|Kokai Maru], was sunk by Allied bombers. Spruance sent in carrier planes again on 29 April to attack Truk, destroying more Japanese air defenses, ground facilities and aircraft. Thereafter, U.S. air units regularly conducted reconnaissance and raids upon Truk until the end of the war, to ensure that it was not restored to its former operational capacity. In Operation Inmate which took place in June 1945, British forces attacked Truk with cruisers and carrier-based bombers to gain Pacific combat experience before more demanding operations against the Japanese home islands. By that time, historian David Hobbs has stated that Truk had been "reduced to starving impotence".
Imperial Japanese Army reinforcements, which had arrived at the atoll before Operation Hailstone, put increasing strain on available foodstuffs and medical supplies. The Truk garrison received few shipments of reinforcements or supplies following the capture of the Palau islands by US forces in September 1944. The garrison's main activity from mid-1944 onwards was growing food to sustain itself. The tropical conditions and damage caused by air attacks complicated this effort, so most of the Japanese personnel became malnourished and ravaged by disease. Nevertheless, the garrison still managed to take extensive measures to protect the atoll from an anticipated U.S. invasion, and placed large stores of food and other supplies in reserve for such an eventuality. Dwindling ammunition even limited the ability of shore batteries to fend off intermittent attacks by Allied forces, including experimental raids by Boeing B-29 Superfortresses and attacks by Allied carrier aircraft. Following the end of the war in August 1945, United States forces found that the garrison still held enough ammunition to supply its gun batteries for at least 30 days of combat.
Truk's garrison formally surrendered at a ceremony conducted on board on 2 September 1945, the same day as the general Japanese surrender documents were signed. The Japanese soldiers and sailors on Truk were repatriated during November and December 1945.
Truk is renowned today as a tourist destination for divers interested in seeing the many shipwrecks left in the lagoon, many of which were sunk in Operation Hailstone.

List of ships in Truk at the time of attack

Warships

List derived from Jeffery's ''War Graves, Munition Dumps and Pleasure Grounds''

Sunk

List derived from Jeffery's ''War Graves, Munition Dumps and Pleasure Grounds''

Sunk

  • Auxiliary cruiser
  • * Aikoku Maru 10,348 tons
  • * Akagi Maru 7,367 tons
  • * Kiyosumi Maru 6,983 tons
  • Navy transport
  • * Hoki Maru 7,112 tons
  • * Yamagiri Maru 7,112 tons
  • * Fujikawa Maru 6,938 tons
  • * Navy transport/freighter San Francisco Maru/Sōkō Maru 5,831 tons
  • * Reiyo Maru 5,446 tons
  • * Seiko Maru ? 5,385 tons
  • * passenger/cargo ship Kensho Maru 4,862 tons
  • * freighter Hanakawa Maru 4,739 tons
  • * passenger/cargo ship Sankisan Maru or Yamakisan Maru 4,776 tons
  • * freighter Hokuyo Maru 4,217 tons
  • * freighter Momokawa Maru 3,829 tons
  • * Navy water carrier/passenger/cargo ship Nippo Maru 3,764 tons
  • * freighter Unkai Maru #6 3,220 tons
  • * Taiho Maru 2,827 tons
  • * freighter Shotan Maru 1,999 tons
  • * freighter Gosei Maru 1,931 tons
  • Freighter Taikichi Maru or Tachi Maru 1,891 tons
  • Army transport
  • * Gyoten Maru 6,854 tons
  • * freighter Nagano Maru 3,824 tons
  • * Yubae Maru 3,217 tons
  • Submarine tender
  • * Heian Maru 11,614 tons
  • * Rio de Janeiro Maru 9,626 tons
  • Oiler
  • * Fleet oiler Shinkoku Maru 10,020 tons
  • * Oil tanker Fujisan Maru 9,524 tons
  • Auxiliary oil tanker
  • * whaler Tonan Maru #3 19,209 tons
  • * Houyou Maru or Hoyo Maru 8,691 tons
  • * passenger/cargo ship Amagisan Maru 7,620 tons

    Damaged

  • Cargo ship Sōya 3,800 tons

    Primary sources

Video

  • Quest for Sunken Warships: "Operation Hailstone", 2007, documentary, Military Channel