This was the US overall scheme for the seizure of key islands among the 100 or more islands of the Palau archipelago as part of the ‘Granite’ plan (spring/summer 1944).
The Palau islands group is located at the western end of the Caroline islands group, some 470 miles (755 km) to the east of the Philippine islands group and 600 miles (965 km) to the north of the western end of New Guinea. The group includes about 100 significant islands with a combined area of 185 sq miles (479 km²), and another 100 or so smaller islets and exposed reefs. The larger islands are mountainous and all are thickly jungled, and some coastal areas are characterised by dense mangrove swamps. There is a large barrier reef off the western end of the island group, this coming close to the shore of Peleliu island, and the east coasts have fringing reefs.
The Japanese took the islands from Germany in 1914, and later annexed them. By 1941 the islands included some 16,000 Japanese civilians, most of whom had emigrated during the 1920s and 1930s, as well as about 6,250 Melanesians, whose lot had improved sufficiently under the Japanese that they were fairly loyal to the empire. More than half of the Melanesian population lived on Babelthuap island, and most of the rest on Angaur island. The Japanese population was concentrated on Koror island, which was the civilian administrative centre for the Japanese mandated islands, and on the islands farther to the south from this.
The group contains several large islands, of which the largest by a very considerable margin is Babelthuap, and there was a large phosphate deposit on Angaur island, producing about 110,000 tons per year during the war, and bauxite deposits throughout the group, producing about 94,000 tons per year. The Japanese had built an airfield on Peleliu and improved the anchorage at Kossol Roads by December 1941. This anchorage served as the mustering point for the 4th Surprise Attack Force. The Japanese had also completed a seaplane and submarine base on Arakabesan isoand just to the west of Koror island. The Japanese later constructed other airstrips on Ngesebus and Babelthuap islands, and a second seaplane base on Koror island.
These islands formed the main southern anchorage of Admiral Mineichi Koga’s (from 3 May 1944 Admiral Soemu Toyoda’s) Combined Fleet from February 1944, and were desired by Admiral Chester W. Nimitz’s Pacific Ocean Areas command as the Japanese air strength on the main islands was being built up from the remnants of Vice Admiral Junichi Kusaka’s 11th Air Fleet largely destroyed over the operational area of General Korechika Anami’s 2nd Area Army in western New Guinea.
Under the overall control of Lieutenant General Hideyoshi Obata’s 31st Army, the Palau islands were garrisoned from March 1944 by one regiment detached from Lieutenant General Shunkichi Ikeda’s 35th Division pending the arrival, during April and May, of Lieutenant General Sadae Inoue’s 14th Division from China. Eventually the islands’ Japanese garrison totalled 21,000 army and 4,000 navy troops, as well as some 10,000 impressed labourers.
The US forces were interested in the Palau islands group not just as a stepping stone from New Guinea to the Philippine islands group, but also as a means of severing the Japanese chain of support for their forces in New Guinea. For this reason the Japanese bases on and round the islands were attacked by US carrierborne aircraft in 'Hailstone' during 17/18 February 1944, and then Vice Admiral Marc A. Mitscher’s Fast Carrier Task Force (three carrier task groups supported by six fast battleships, 13 cruisers and 26 destroyers) of Vice Admiral Raymond A. Spruance’s 5th Fleet struck at the islands again on 30 March/1 April in preparation for the US Army’s ‘Reckless’ operation against Hollandia in New Guinea.
For this latter, three carrier tasks groups sortied from Majuro atoll on 22 March, together with two support groups, and streamed a circuitous passage to the south of Truk atoll. The US forces were nonetheless sighted by a Japanese reconnaissance aeroplane on 25 March. Spruance responded by speeding the schedule for the operation, ordering his destroyers to refuel from the support group on 28 March and racing in to attack Kossol Roads two days earlier than planned. This was not enough, for the forewarned Japanese fleet was able to make good its escape. The Americans did nonetheless destroy 30 Japanese aircraft and sink 28 merchant ships totalling 129,807 tons as well as the destroyer Wakatake, and the super-battleship Musashi was torpedoed and lightly damaged by the US submarine Tunny. The operation was marked by carrierborne aircraft’s first use of aerial mines, of which 78 were laid by a specially trained squadron of Grumman TBF/TBM Avenger warplanes in the approaches to the main anchorage, and more than 30 merchant ships were trapped. Subsidiary attacks were also directed at Yap and Woleai islands during this raid.
Koga was immediately impressed with the newly revealed vulnerability of his Combined Fleet’s main anchorage, and ordered a withdrawal to Tawi-Tawi, between Borneo and the Philippine islands group.