From Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations Against Japan, 1941-45 (The Secret War), by John Winton (Sapere Books, 2022), Kindle pp. 225-227:
Known as the ‘Gibraltar of the Pacific’, Truk was in fact not nearly as strongly defended as legend had it, but it was the best fleet anchorage anywhere in the Japanese mandated islands and had been the regular base for the Combined Fleet since July, 1942.
Truk’s geographical layout, of scattered volcanic islands inside a triangular-shaped coral reef, made it virtually impregnable to surface attack from outside its perimeter. But it was open to the air. After the same FRUPAC analysis of air search patterns from Truk as from Kwajalein, a powerful task force including six fleet and four light carriers in three groups under Mitscher (who had relieved Pownall in January) made a fast and undetected run towards Truk in the night of 16/17 February, 1944, to carry out Operation HAILSTONE.
Spruance himself was present, flying his flag in the battleship New Jersey (the fleet commander had also been present during the Marshalls landings, ready to take over command if the Combined Fleet sallied out). But there was no chance of that. Truk was just within bombing range of Kwajalein and Bougainville, and its supply route from the Empire was constantly beset by US submarines. Thus, Truk was no longer the safe base it had once been.
Photo-reconnaissance of Truk on 4 February showed plenty of targets, but the same reconnaissance flight had given the game away to Admiral Koga, who sent most of his warships to Palau and went back to Japan himself in the giant battleship Musashi.
The last cruiser, Agano, left Truk on 16 February and was torpedoed and sunk by the submarine Skate the next day. But the fleet auxiliaries, the oilers, seaplane carriers, submarine tenders and many Marus of the support force were due to leave later and were still in Truk when the first fighter sweep, launched from a point 90 miles north of Truk before dawn on 17 February, caught the defenders by surprise.
Seventy-two fighters followed by eighteen Avengers with incendiaries found some fifty merchant ships in the harbour and 365 aircraft ranged on the airfields. The strike put all but a hundred of the aircraft out of action. The carriers then mounted more or less continuous strikes of fighters, bombers and torpedo-bombers to work over the airstrips and attack shipping. That evening the Japanese made their only reply, a torpedo attack by Kates who scored a hit on the carrier Intrepid (a somewhat unlucky ship, nicknamed ‘The Evil I’), putting her out of action for some months.
In the meantime Spruance in New Jersey with another battleship, Iowa, two heavy cruisers, four destroyers and the light carrier Cowpens to give air cover, made one anti-clockwise sweep round Truk to catch any would-be escapers. They sank the light cruiser and Sixth Fleet submarine flagship Katori and the destroyer Maikaze.
That night a strike of Avengers, specially equipped and trained for night bombing, attacked shipping in the lagoon. It was the first time in the war such a raid had been made and it was a signal success: one-third of the total tonnage destroyed at Truk was sunk by these Avengers.
Strikes resumed the next day, 18 February. Everything that moved or floated had now been sunk or strafed and the aircraft turned their attention to fixed fittings — hangars, fuel tanks, storage dumps, buildings and vehicles. When the carriers retired at noon their aircraft had flown 1,250 sorties, dropped 400 tons of bombs and torpedoes on shipping and 94 tons on airfields and shore installations. They had sunk the cruiser Naka, auxiliary cruisers Aikoku Maru and Kiyosumi Maru, destroyers Oite, Fumizuki and Tachikaze, the armed merchant cruiser Akagi Maru, two submarine tenders, an aircraft ferry, six tankers and seventeen other ships — a total of about 200,000 tons. This was a crushing blow to the Japanese Navy. The loss of so many fleet supply and support vessels was as grave an operational defeat as the loss of capital ships. Truk was never the same again.