The Battle of the Hongorai River took place during the Second World War and involved Australian, New Zealand and Japanese forces. Part of the wider Bougainville Campaign of the Pacific theatre, the battle was fought in the southern sector of Bougainville Island. Coming after the Battle of Slater's Knoll in which a strong Japanese counterattack was defeated, the battle occurred in two distinct periods between 17 April and 22 May 1945, as elements of the Australian 15th Brigade advanced south along the Buin Road.
The initial phase saw the Australians advance towards the Hongorai River. Following the end of the early fighting, the Australian advance towards the main Japanese concentration at Buin continued as they struck out towards the Hari and Mivo Rivers. This continued until torrential rain and flooding brought the advance to a halt short of the objective, washing away many bridges and roads upon which the Australians relied for supplies. As the Australian advance stalled, the Japanese began harassing the Australian line of communications, and as the rain stopped and the flooding subsided in late-July and into August, the Australians began making preparations to resume the advance towards Buin again. Ultimately, though, the war came to an end before the final Australian advance began, bringing the campaign to an end.
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 After an initial counter-attack, the US Marines had been replaced by a garrison of US Army troops who began consolidating their position around Torokina, establishing a strong perimeter.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, as part of plans to free US troops up for the Philippines campaign, the Australian II Corps—consisting of mainly Militia troops under the command of Lieutenant General Stanley Savige—took over responsibility for Allied operations on Bougainville from the American XIV Corps. Australian forces began arriving on the island between November and December 1944, initially establishing themselves around the US base at Torokina. Due to inaccurate intelligence, Savige mistakenly believed that the Japanese forces on the island numbered just 17,500 men, and he consequently decided that the Australians would pursue an aggressive campaign to clear the Japanese from Bougainville in order to free their troops for subsequent operations elsewhere, rather than maintaining the defensive posture the US forces had adopted. However, Allied estimates of Japanese strength were later found to be grossly inaccurate and after the war it was found that the number of Japanese on the island at this time was closer to 40,000.
The campaign that Australian planners developed entailed three separate drives: in the north, it was planned that Japanese forces would be forced into the narrow Bonis Peninsula and contained; in the centre the seizure of Pearl Ridge would give the Australians control of the east–west avenues of approach, as well as affording them protection against further counter-attacks, while also opening the way for a drive to the east coast; and the main campaign in the south, where the bulk of the Japanese forces were concentrated around Buin.