Truk Lagoon: History
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As early as 1918, the Provisional South Seas Defense Force initiated numerous surveys of the islands for their potential as naval bases as the Japanese navy recognized that the Mandate Islands were of great strategic importance and planning was needed for their rapid militarization in the event of war with the United States. Throughout the 1920s, Japan’s policy of secrecy with respect to the islands including the exclusion of visits by foreign ships perpetuated the suspicions among Western powers that she was preparing the islands for war. In reality, Japan was concentrating more towards consolidating its colonization footholds and making a profit during these years and all efforts were oriented towards that end. In 1930, American, Japan, and British delegates met at the London Navy Treaty conference to revise and update naval arms limitation agreements that had initially been set eight years earlier in Washington. Japanese proposals, presented by Rear Admiral Isoroku Yamamoto, for more equitable apportioning of naval ship building quotas were argued against vehemently by the American and British delegates. Following an intense struggle in a confrontational atmosphere with numerous compromises, the Japanese representatives signed the treaty agreement. Although the new warship building quotas agreed to swing in the direction of Japanese objectives, naval hard-liners in Japan heartily denounced the new treaty provisions and called for Japan’s withdrawal from the naval treaty system. This reaction brought about increased fears of resurgent Japanese imperialism and questions about their intentions to build up their fleet and fortify the Mandate Islands. The only transformation of the islands that could be construed as preparations for military purposes through the 1930-34 time period was the construction of the commercial and communications facilities by the South Seas Government and the South Seas Development Company.
In 1934, construction began on the airfield on Eten Island. This project, necessitating the leveling of half of the island for the airstrip using only dynamite and manual labor would take nearly seven years to complete. After the expiration of the Washington Treaty with its naval limitations system non-fortification clauses in 1937, further communications facilities and air base construction was begun jointly between the Japanese navy and South Seas Government following the dispatch of the seaplane tender Kamoi with a naval engineer, Naoyoshi Itsumi, to survey the islands further for potential military sites. By 1939, plans were implemented for the intensified construction of naval and airfield facilities. Japan passed the Military Manpower Mobilization Law in at this time that provided for conscripted labor from the Empire. In addition, large numbers of Korean coolies were shipped to the mandates and local islanders were conscripted for heavy construction work. Further help was provided in October 1939 when nearly two thousand Japanese convicts, mainly from Yokohama Central Prison, were organized into penal labor battalions and shipped to the islands. Many of these convicts were sent to Truk where they were used principally for airfield construction on Moen Island. The Fourth Mandate Fleet was organized on November 15, 1939 as a Holding Force in the Central Pacific Fleet organization under orders of the Combined Fleet. Its primary mission was to protect the Mandated Islands area, now termed the Inner South Seas (uchi Nan ‘yo). Under command of the Fourth Fleet was the Fourth Base Force (“konkyochitai”) at Truk which controlled all naval garrisons and installations in the Eastern Caroline Islands.
You can read more about this in Dan Baileys book WWII Wrecks of Truk Lagoon