Japanese forces had landed on Bougainville in early 1942 , capturing it from the small force of Australians garrisoning the island . They had subsequently developed several airbases on the island , using it to conduct operations in the northern Solomon Islands and to attack the Allied lines of communication between the United States , Australia and the Southwest Pacific Area . These bases also helped protect Rabaul , the major Japanese garrison and naval base in Papua New Guinea , and throughout 1943 , Allied planners determined that Bougainville was vital for neutralising the Japanese base around Rabaul .