The Bougainville campaign (Operation Cherry Blossom) was fought by the Allies in the South Pacific during World War II to regain control of the island of Bougainville from the Japanese forces who had occupied it in 1942. During their occupation the Japanese constructed naval aircraft bases in the north, east, and south of the island; but none in the west. They developed a naval anchorage at Tonolei Harbor near Buin, their largest base, on the southern coastal plain of Bougainville. On the nearby Treasury and Shortland Islands they built airfields, naval bases and anchorages. These bases helped protect Rabaul, the major Japanese garrison and naval base in New Guinea, while allowing continued expansion to the south-east, down the Solomon Islands chain, to Guadalcanal.