Operation Hailstone

Operation Hailstone (, lit. "the airstrike on Truk Island"), 17–18 February 1944, was a massive United States Navy air and surface attack on Truk Lagoon conducted as part of the American offensive drive against the Imperial Japanese Navy (IJN) through the Central Pacific Ocean during World War II.
Prior to Operation Hailstone, the IJN had used Truk as an anchorage for its large Combined Fleet. The coral atoll surrounding Truk's islands created a safe harbor where the few points of ingress and egress had been fortified by the Japanese with shore batteries, antiaircraft guns, and airfields.
American estimates of Truk's defenses and its role as a stronghold of the Japanese Navy led newspapers and military men to call it the "Gibraltar of the Pacific", or to compare it with Pearl Harbor. Truk's location in the Caroline Islands also made it an excellent shipping hub for armaments and aircraft moving from Japan's home islands down through the South Seas Mandate and into the Japanese "Southern Resources Area".
By early 1944, Truk was increasingly unsustainable as a forward base of operations for the Japanese. To the west, American and Australian forces under General Douglas MacArthur had moved up through the Southwest Pacific, isolating or overrunning many Japanese strong points as part of Operation Cartwheel. The U.S. Navy, Marine Corps, and Army, under the command of Admiral Chester W. Nimitz, had overrun the most important islands in the nearby Gilbert Islands and Marshall Islands, and then built numerous air bases there.
As a result, the Japanese Navy had to relocate the Combined Fleet's forward base to the Palau Islands, and eventually to Indonesia, and the Fleet had begun clearing its major warships - carriers, battleships, and heavy cruisers - out of Truk before the "Hailstone" attack struck.
Nevertheless, the "Hailstone" attack on Truk caught a good number of Japanese auxiliary ships and cargo ships in the harbor, as well as some smaller warships. Between the air attacks and surface ship attacks over the two days of Operation Hailstone, the worst blow against the Japanese was about 250 warplanes destroyed, with the concurrent irreplaceable loss of experienced pilots, and 17,000 tons of stored fuel. Also, about forty ships — two light cruisers, four destroyers, nine auxiliary ships, and about two dozen cargo vessels — were sunk.
Considerable damage was inflicted on the various island bases, including dockyards, communications centers, supply dumps, and its submarine base. Truk remained effectively isolated for the remainder of the war, cut off and surrounded by the American island hopping campaign in the Central Pacific, which also bypassed important Japanese garrisons and airfields in the Bismarck Archipelago, the Caroline Islands, the Marshalls, and the Palaus. Meanwhile, the Americans built new bases from scratch at places like the Admiralty Islands, Majuro, and Ulithi Atoll, and took over the major port at Guam.
Background.
The Japanese occupied Micronesia, including the Caroline Islands, in 1914, and established Truk as a base as early as 1939. The lagoon was first built up to house the IJN'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.
Despite the impressions of U.S. Navy leaders and the American public concerning Truk's projected fortifications, the base was never significantly reinforced or protected against land attack. In fact, the development of Truk only began in earnest and in hurried fashion in late 1943, when the airfields were extended, shore batteries were erected, and other defensive measures taken against a U.S. invasion.
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 A. 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 (TGs) were committed to the operation. Their total strength consisted of five fleet carriers
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and four light carriers (the
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), carrying a total of 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 fleet units from its anchorages 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.
Attack.
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. 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 there were more than 300 Imperial Japanese Navy Air Service (IJNAS) and Imperial Japanese Army Air Service (IJAAS) planes 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 and surprise, achieved a one–sided victory against IJNAF pilots flying the inferior Mitsubishi A6M Zero. As many as 30 of the 80 Zeros sent up in response to the fighter sweep were shot down, compared with four Hellcats reported lost. 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. Still others, 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 carrier air groups (CAGs) 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 "Enterprise"s Air Group 10 (CAG-10) and "Intrepid"s CAG-6 dropped fragmentation and incendiary bombs on runways at Eten Island as well as the seaplane base on Moen Island. Dozens of aircraft were damaged or destroyed, 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 (later Rear Admiral) James D. Ramage, commanding officer of Dive Bombing Squadron 10 (VB-10), is credited with sinking the previously damaged merchant tanker "Hoyo Maru". Lieutenant James E. Bridges and his crew in one of "Intrepid"s Torpedo Squadron 6 (VT-6) Avengers scored a direct hit on the ammunition ship . The bomb blast set off a tremendous explosion, which immediately sank the ship and apparently engulfed the plane as well, killing all three men inside.By the second and third anti-shipping strikes of the day, carrier air group action reports listed the apparent enemy mission as "escape". Those ships 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", "Nowaki" and minesweeper "Shonan Maru"—was given special attention by carrier bombers. Multiple air groups attacked these ships, inflicting serious damage. "Yorktown"s dive and torpedo bombing squadrons claimed two hits on "Katori" and hits on another cruiser and multiple destroyers; "Essex" bombers claimed five hits on a as well, stating that the ship was stopped dead in the water after the attack.
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 air groups to stop attacking "Katori" and her companions. The admiral 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 incurred 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. The IJN lost "Maikaze", "Shonan Maru", "Katori" and "Akagi Maru". Destroyer "Nowaki" was the only Japanese ship from this group to escape.
Retaliation for the day's strikes arrived late at night in the form of small groups of Japanese bombers 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 such plane, a Nakajima B5N2 "Kate" bomber, managed to evade night fighter planes protecting the U.S. task force and dropped its torpedo on Task Group 58.2. The torpedo struck "Intrepid" on the starboard quarter of the ship, 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.
Aftermath.
Truk, like so many other Japanese bases, was left to itself without hope of resupply or reinforcement. Army forces which had arrived at the atoll before the U.S. attacks put increasing strain on available foodstuffs and medical supplies. 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.
Losses at Truk were severe. Some 17,000 tons of stored fuel were destroyed by the strikes. Shipping losses totaled almost 200,000 tons including precious resources in fleet oilers. This represents almost one tenth of total Japanese shipping losses between 1 November 1943 and 30 June 1944. Moreover, the isolation of this whole area of operations by submarine and air attack began the effective severance of Japanese shipping lanes between empire waters and critical fuel supplies to the south. The ultimate effect of such a disconnect was later seen during the Battle of Leyte Gulf, when IJN forces had 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 for the first time put U.S. land-based heavy bombers within range of the Japanese home islands.
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" (2007)
Merchant ships.
List derived from Jeffery's "War Graves, Munition Dumps and Pleasure Grounds" (2007)