The Japanese had invaded Bougainville in early 1942, and had established several airfields on the island, with key bases being constructed around Buka, at Kahili and Kieta, and on the Bonis Peninsula. From these bases, the Japanese had struck south towards Guadalcanal, in an effort to sever sea lanes of communication between the United States and Australia. Japanese air and naval movements around Bougainville had been monitored by a small group of Allied Coastwatchers, who were able to gain considerable intelligence through the native population of the island. However, by early 1943, with the conclusion of the Guadalcanal Campaign in the Allies' favor, and a string of defeats in the central Solomons, the Japanese had sought to consolidate their hold on Bougainville. They had slowly cleared the Coastwatchers from Bougainville, with the remaining personnel being withdrawn by the US submarine in March 1943.