From “Importance of Japanese Naval Bases Overseas,” by Masataka Chihaya (written on 14 January 1947), in The Pacific War Papers: Japanese Documents of World War II, edited by Donald M. Goldstein and Katherine V. Dillon (Potomac Books, 2006), pp. 63-65 (paraphrased freely):
Truk Lagoon is one of the greatest, forming a rough triangle more than 30 miles on each side. Inside the lagoon are many islands, not sand islands or coral reefs, of which more than eight are more than one square mile in size. It provided not just sufficient anchorage for a whole Japanese fleet during that era, but also enough area to allow several vessels to maneuver for training. The islands also provide enough room for several airstrips. In fact, by the end of the war, the Japanese Navy had built at least four such strips. The climate is also tropical but mild. In addition to these advantages, Truk occupies a key position in the middle Pacific area, able to control Midway to the north, the Marshalls in the east, and Rabaul and New Britain to the south. From every point of view, Truk was one of Japan’s most important bases in the Pacific.
When it occupied Micronesia in World War I, and during the League of Nations mandated administration that followed, the Japanese Navy was well aware of Truk’s importance. However, in strict observance of the postwar naval treaties, it did little to establish a naval base there. It may be hard to believe, but it is true. At the outbreak of the Pacific War there was only one half of a completed airstrip on Takeshima (Bamboo Island), a small island less than 1,000 meters long. There was no underground oil storage, nor any repair facilities on land. The only naval facility worthy of the name was that half-completed airstrip.
Even after war broke out, the Japanese Navy was rather slow to strengthen Truk Naval Base. As soon as the U.S. Navy began its offensive on Guadalcanal in the summer of 1942, and the Solomon Islands became the main theater of fighting for both navies, Truk became the center of Japanese naval operations. Almost all naval vessels gathered there before making sorties into the Solomons, and returned there for refueling and repair when damaged. Never before had the need for oil storage and repair facilities been more urgent, and the Japanese Navy concentrated its oil tankers and repair ships there while quickly trying to build such facilities on land as well. But it was too late. This concentration of oil tankers in Truk disrupted the flow of oil from Southeast Asia back to the Japanese homeland. Even the giant battleships Yamato and Musashi, which should have been at the center of the Japanese fleet, were often nicknamed “the tankers Yamato and Musashi” because they served as tankers supplying fuel to smaller warships instead of engaging in combat operations.
As there were not enough repair facilities on Truk, Japanese naval vessels sometimes had to go all the way back to the homeland for repairs, thus reducing the size of the naval forces available for the Solomons campaign.
It was not until the summer of 1943 that the Japanese Navy began to construct three more airstrips at Truk, two on Harushima (Spring Island) and one of Kaedeshima (Maple Island). By the time the U.S. Navy made a surprise attack on Truk on 17 February 1944, those three bases were almost complete, but they lacked adequate radar and command-and-control equipment, which would have made them more useful.
As a result, during the surprise attack on Truk in February 1944, U.S. Navy carrier-borne aircraft came out of the blue, destroying one light cruiser, 4 destroyers, 26 transport ships, 3 oil tanks, 2,000 tons of food, and more than 180 airplanes, of which more than 100 were lost on the ground.
This fiasco, together with the loss of the Gilberts and Marshalls, suddenly lessened the importance of Truk as a naval base. The Japanese fleet, which had long gathered at Truk, moved westward into the Carolines, Singapore, and even the homeland. Soon afterward, the Japanese Navy withdrew its land-based aircraft to the Marianas and western Carolines. Truk was no longer a vital naval base, just a stepping stone between the Marianas and Rabaul.
The bad situation on Truk got worse when U.S. forces invaded the Marianas in June 1944. Truk could contribute little to the Japanese defense, and the fall of the Marianas left Truk largely isolated, except for very few small visits by submarines and flying boats. From that time on, Japanese forces on Truk had to endure not just Allied air attacks, but mounting starvation and disease until the war ended in August 1945.