The Battle of Ratsua occurred during the Second World War and involved Australian and Japanese forces. Part of the wider Bougainville Campaign of the Pacific theatre, the battle took place in the northern sector of Bougainville between June and August 1945. The main forces that took part in the fighting were the Australian 23rd Brigade and the Japanese 87th Naval Garrison Force. The fighting occurred following a failed landing by Australian forces at Porton Plantation in early June, which was part of an attempt to outflank a series of strong points that the Japanese had built across the base of the Bonis Peninsula. After this, the fighting in the northern sector around Ratsua essentially became a containment action for the Australians as they concentrated their efforts and resources upon driving south towards Buin, which was the main centre of Japanese resistance on the island at the time. As a result, the situation around Ratsua remained largely static until the end of hostilities in mid-August 1945. Background Strategic situation 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. US Marines conducted an amphibious landing at Cape Torokina, on the western coast of the island, north of Empress Augusta Bay, in November 1943. After an initial counter-attack, the US Marines had been replaced by a garrison of US Army troops who began consolidating their position around Torokina, establishing a strong perimeter. In March 1944, the Japanese launched a heavy counter-attack, which was turned back with heavy casualties. After this, the situation on Bougainville became largely static, as the Japanese focused primarily on subsistence, and the US forces chose to adopt a mainly defensive posture focused on maintaining the perimeter around Torokina. In late 1944, the Australian Army had assumed responsibility for Allied operations on Bougainville, replacing US troops who were subsequently redeployed to the Philippines. At the time, the Australians believed that there were only 17,500 Japanese on the island,