On 31 January 1944, Kwajalein and Majuro were invaded. Kwajalein was captured in four days. On Majuro, a secret chart of all Japanese mines and swept channels was captured facilitating the clean up. On Kwajalein, a complete call sign listing of all Japanese ships and shore stations was captured along with other communications documents.
Navy radio intelligence determined that there was a large buildup of the First Air Fleet aircraft in Iwo Jima, Marianas and Philippines with its headquarters at Tinian. Carrier raids were carried out against the Marianas and Truk. In response to urgent requests from MacArthur, Palau and Hollandia were hit by TF58 in March and April. Advance warning by the Japanese radio intelligence headquarters at Owada Communications Unit in Japan allowed the heavies of the Combined Fleet to escape Truk and Palau, although many oilers, tenders and supply ships were sunk at Truk. CinC of the Combined Fleet, Admiral Mineichi Koga’s flight from Palau to Davao was never heard from. Admiral Soemu Toyoda was appointed as the new CinC of the Combined Fleet. Chief of Staff Shigeru Fukudome’s plane crashed in the Philippines and he was captured by a guerilla group. He though the documents he was carrying, including the Decisive Plan, went to the bottom, but a fisherman retrieved the briefcase. Later, it was sent to Australia by submarine. Meanwhile, Fukudome was delivered to a Japanese Army search party to prevent reprisals. To protect MacArthur’s flank, Marines and Army troops captured Palau with the former suffering a high percentage of casualties.
As the large forces of seven fleet carriers, eight light carriers, seven fast battleships, ten cruisers and sixty destroyers converged on the Marianas, Ensign C.A. Sim’s RIU aboard the Indianapolis determined they had been sighted by a search aircraft. This caused Mitscher to launch a fighter sweep earlier than planned to clear the skies of land based enemy aircraft. Attacks on Saipan, Tinian, Rota and Guam followed. On June 16, troops landed on Saipan with a hundred ships offshore. Part of the IJN’s defense was a submarine patrol line west of the Marianas. When this line was moved 60 miles southeast due to an aircraft spotting report, that instruction was decrypted by FRUPAC. A new hunter-killer group of destroyer escorts was directed to “roll up” this line of submarines. In twelve days, England recorded six submarine kills, five of which were assisted by the Ultra intelligence provided. Subsequently, the Japanese suspected the losses were due to radio intelligence and changed their operating procedure from patrol lines to patrol areas. Thus, most of the land based aircraft and the submarines, two parts of the Decisive Plan, had been disposed of before the Japanese carrier planes made their belated attack on U.S. fleet units.
Both FRUPAC and Washington accurately predicted Japanese responses to the Saipan invasion including Ozawa’s six carriers and an array screening vessels. These reports were confirmed by submarine sightings as the units moved through the San Bernardion Straits and again on 17 June 1944 when they were 800 miles west-southwest of Saipan and closing. On the night of 18-19 June, Ozawa broke radio silence to coordinate his attack with the First Air Fleet’s land-based bombers (not knowing they were already heavily damaged) and to inform Toyoda of his intentions. This transmission was fixed by the Pacific HFDF Net and reported to Mitscher. Then, the three RIU units on Indianapolis, Hornet and Yorktown monitored the Japanese carrier aircraft circuits when they became active about 0600. By 0800, 50 to 60 aircraft were noted airborne and the U.S. forces were alerted that they were on their way based on a sighting report of U.S. fleet units from one of Ozawa’s scouting planes. In addition to excellent fighter director information obtained from radar on the altitude and bearing of enemy attack aircraft, the three RIU language officers monitored the enemy’s flight senior squadron commander directing aircraft strikes by voice and passed that information along to fighter interceptor officers. As a result, only one Japanese group reached a position to attack U.S. carriers. So many aircraft were downed, the action was aptly named, “The Great Marianas Turkey Shoot.” RIU’s followed U.S. attack aircraft from sighting reports on them made by Japanese ships and aircraft and reported them to their commanders. Between two carrier aircraft attacks and the submarine Cavalla, the Japanese lost three fleet carriers and two oilers.
An intelligence haul of some fifty tons of documents was acquired from Saipan alone. One recovery was the Jade cipher machine (an upgraded version of the Purple cipher machine with katakana characters) used by the IJN. An operator was in the process of destroying it when he was shot. Weeks later, Jade, which had other problems, was discontinued.
Guam was secured on 10 August. Subsequently, a forward echelon of CinCPac and an advanced intercept and processing station with Hypo personnel were established there. This station, almost two thousand miles closer to Japan proper, could now insure most of the Japanese naval and naval air communications would be intercepted and processed on a timely basis.
While information from FRUPAC decrypts were first provided to the B-29 20th Bomber Command in India through a navy liaison officer in the spring of 1944, this information continued to be supplied when the B-29’s moved to the Marianas.
The next assault was made on Iwo Jima, primarily to reduce B-29 casualties by providing an emergency landing field between the Marianas and Japan. By this time, U.S. fleet forces were so powerful no Japanese surface forces even attempted to interfere with the the landing or support ships. Kamikaze suicide missions became more frequent and one sunk the escort carrier Bismark Sea and damaged the carrier Saratoga at Iwo. The fleet carrier Randolph was hit at Ulithi in a nighttime attack by two engine bombers from Truk. In addition to installing the usual HFDF station on the newly conquered Iwo Jima, a small very high frequency (VHF) voice intercept site was also set up there to provide early warning information on Japanese aircraft operating in the area.
On 1 April 1945, Allied forces invaded Okinawa. In its last surface force gasp, the Imperial Naval General Staff ordered the super battleship Yamato, the light cruiser Yahagi and six destroyer to make a suicide foray against U.S. fleet units supporting the invasion. FRUPAC decrypted the entire sortie plan on 5 April and Admiral Spruance ordered his battleship and carrier commanders, “You take them on” some fifteen hours before the Yamato got underway. U.S. carrier aircraft stopped the “unsinkable” Yamato dead in the water by a single torpedo. Five more torpedo hits finally sunk the super battleship. The Yahagi and four destroyers also went down to Davy Jones’ locker from hits by carrier aircraft.
However, U.S. fleet units did not escape the terrific pounding from about 700 attacks from Japanese aircraft including some 355 by kamikazes on 6 and 7 April. As soon as the flight leaders were airborne, Spruance’s RIU would give him the estimated time of arrival so he could order the fleet to maneuver accordingly. Thus, only two destroyers were sunk but many ships were damaged. On April 12, another destroyer was sunk and many vessels were damaged including Spruance's flagship, the battleship New Mexico and the Enterprise. A member of the RIU aboard the New Mexico was killed when a kamikaze hit the ship's smokestack.The attacks from kamikazes continued for four months. In total, thirty-four destroyers and smaller ships were sunk while 368 warships were damaged including eight carriers, four escort carriers, ten battleships, five cruisers, and 63 destroyers. While some 7,000 soldiers and marines were killed in the conquest of Okinawa, 5,000 sailors lost their lives at sea.
As the Allies
planned for the invasion of Japan, intelligence determined that the Japanese
reserved as many as 5,000 planes and hoarded aviation gasoline to fight
the battle for the homeland. Some 900,000 troops were dug in on Kyushu,
the next target, and millions more were being mobilized on the main island
of Honshu. Fortunately for both sides, the dropping of two atomic
bombs convinced the Emperor to agree to an unconditional surrender, which
was formally signed on 2 September 1945. Thus, far more Japanese
lives were saved than were lost in the two atomic blasts not even considering
the number of Allied lives spared.