The Japanese defence of the Admiralties fell under the Eighth Area Army, based at Rabaul and commanded by General Hitoshi Imamura. In September 1943, as a result of the failure to stop Allied advances in New Guinea and the Solomons, the Imperial General Headquarters (IGHQ) had decided to constrict Japan's defensive perimeter in the south and central Pacific to a new line stretching from the Banda Sea to the Caroline Islands. The IGHQ charged Imamura with holding his portion of the new line, which included the Admiralties, as long as possible to allow the Japanese navy and army time to prepare "decisive" counterattacks against Allied forces. Maintaining control of the Admiralties was crucial to the Japanese defensive plans, as possession of the islands by the Allies would place the key Japanese stronghold at Truk within range of heavy bombers. Apparently not expecting the Allies to move on the Admiralties so quickly, IGHQ gave Imamura until the middle of 1944 to complete the defensive preparations for his command. At this time the largest Japanese unit in the islands was the 51st Transport Regiment, which had arrived on Los Negros in April.