The Battle of Tsimba Ridge was an engagement of the Second World War involving Australian and Japanese forces. Part of the wider Bougainville campaign of the Pacific theatre, the battle occurred in the northern sector of Bougainville Island between 17 January and 9 February 1945, when the lead battalion of the Australian 11th Brigade—the 31st/51st Infantry Battalion—advanced towards the Genga River as part of a drive north to the Bonis Peninsula.
On the peninsula, an enclave of Japanese troops had established itself after withdrawing north following earlier engagements in the centre of the island throughout 1944–45. Coming up against heavy resistance along Tsimba Ridge, the Australians' advance was halted for three weeks as they fought to wrest control of the ridge from the defending Japanese from the 81st Infantry Regiment, whose strength was roughly equal to that of the Australians. The main Australian attack took place on 6 February, but it was not until 9 February that the Japanese completely withdrew from the position. Small-scale fighting in the surrounding area continued until 22 February. The next stage of the campaign saw the Australians take Ratsua as they sought to establish a defensive line across the base of the Bonis Peninsula between Ratsua and Ruri Bay.
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; 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 Bougainville, north of Empress Augusta Bay, in November 1943. After an initial counter-attack that was repulsed, the marines had been replaced by a garrison of US Army troops who began consolidating their position around Torokina, establishing a strong perimeter.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 many 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 mostly defensive posture aimed at maintaining the perimeter around Torokina.
In November–December 1944, the Australian II Corps—consisting of militia troops from the 3rd Division and the 11th Brigade—under Lieutenant General Stanley Savige took over responsibility for Allied operations on Bougainville from the American XIV Corps as part of a plan to free up the US troops for the Philippines campaign. Mistakenly believing that the Japanese forces on the island numbered just 17,500 men, it was decided that the Australians should pursue an aggressive campaign to clear the Japanese from Bougainville. After taking over the US perimeter that had been established around Torokina, the Australian campaign developed into 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. Allied estimates of Japanese strength were later found to be grossly inaccurate and, after the war, it was found that the Japanese alive on the island at this time numbered closer to 40,000.
Preliminary moves On 31 December 1944, Savige ordered Brigadier John Stevenson, commander of the 11th Brigade, to begin clearing operations along the northwest coast of the island towards Soraken Harbour, as the Australians attempted to drive the Japanese in the sector on to the narrow Bonis Peninsula. At the time the Australians believed there were around 390 Japanese defending the area surrounding the Genga River—considered to be the main Japanese defence line in the northern sector—but it was later determined to be closer to 900 men from the 81st Infantry Regiment, under the command of Lieutenant Colonel Shinzo Nakamura.
Nakamura, who, according to Peter Charlton, was considered to be a "master of jungle warfare", had been ordered to prevent the Australian advance along the coast. He had perceived the likelihood of stopping the Australians well forward of the Genga as being unlikely and had therefore decided that he would concentrate his defence around the river itself, establishing a strong position on Tsimba Ridge, about south of the river. Described by war correspondent Fred Aldridge as a "horseshoe volcanic feature with steep jungle-clad sides...each toe [of the ridge ended] close to the sea, forming a natural amphitheatre by the beach", it was a "textbook fortress". According to Aldridge, it was a "maze of dugouts, trenches, [and] foxholes", and was well camouflaged and heavily defended by snipers, machine guns and artillery, while supporting elements had been placed to the north, in depth, behind the Genga, near Kunamatoro.
Taking over control of the northern sector from the 7th Brigade, which was subsequently transferred to the southern sector to take part in the main offensive, the 11th Brigade advanced along the coast from Kuraio Mission, sending out patrols into the interior to flush the Japanese out of their positions in the mountains as they went. The 31st/51st Infantry Battalion, under the command of Lieutenant Colonel Joseph Kelly and roughly 800-strong, formed the vanguard of the advance, its main body moving along the coast road towards Amun, Sipaai and Rukussia, while patrols were sent inland from Totokei towards Lalum and Soraken, both of which were considered key Japanese positions. By 16 January 1945, the lead company of the 31st/51st reached Rukussia, having encountered no Japanese up until that point.
Battle
The first contact came on 17 January when the 31st/51st Infantry Battalion's advanced elements—drawn mainly from 'D' Company, under Captain Thomas Titley—moved north from Rukussia and proceeded towards the Genga River. Reaching a clearing north of Puto, a platoon-sized patrol came across several huts in a cleared area and proceeded to attack it. One Japanese was killed in the initial assault but, as the Australians took the huts and pushed beyond them, the Japanese engaged them from entrenchments in front of them. The Japanese then launched a counter-attack using light and heavy machine guns. In response, the Australians brought up another platoon and, over the course of two days, repulsed the counter-attack, with six to eight Japanese estimated to have been killed and others wounded, while two Australians were wounded.
During the engagement, the Japanese had opened up with the artillery that they had brought up in support of the main defensive position on Tsimba Ridge, firing for the first time in the sector. This barrage was largely ineffective, inflicting only one casualty, as many of the shells proved to be faulty and did not detonate. The Australians also came across a large camp, including a field hospital, which they captured intact. Another group of huts was also discovered, from where they gained intelligence identifying the Japanese troops in the area as belonging to the 81st Infantry Regiment. They also managed to capture two 70 mm guns and ammunition for them.
On 19 January, the 31st/51st Infantry Battalion's 'A' Company, under Captain Clyde Downs, sent a platoon-sized patrol out to outflank the Japanese forward positions; this patrol moved first to the right flank and then north along the jungle tracks around Totokei and Goton. Discovering evidence of Japanese movement in the area, they proceeded to the village of Kunamatoro, about north of Junapopo. Here they encountered a force of around 30 to 50 Japanese and, after the initial contact, in which the Australian platoon commander was wounded, the patrol attacked the village. Within 10 minutes the battle was over: 14 Japanese had been killed and five wounded, while one Australian was killed, one wounded, and one missing. As darkness fell, the Australian patrol withdrew from the village.
The following day—20 January—the advanced elements of the Australian force pressed on towards Tsimba Ridge. A formation about long and about in height, and stretching westerly to the Genga River where a steep cliff fell to the southern bank of the river, the ridge blocked the Australian advance across the river, while its south-easternmost protrusion (later known to the Australians as the "Pimple") was separated by a shallow saddle. On top of the ridge, dense jungle provided good concealment, while most of the approaches to the ridge were open to observation. As the Australians approached the ridge, one of their platoons captured a 47 mm gun positioned along the coast about south of the ridge, while another platoon contacted a lone Japanese soldier, who was killed in the subsequent firefight as they entered a garden that curved along the southern approach to the ridge, and which became known as the "Amphitheatre". After the initial contact, the platoon's advance through the garden was prevented by an intense burst of fire from the ridge, but the Australians managed to scale the high ground that rose to their east and were able to occupy the south-eastern tip of the Pimple, capturing two 75 mm guns in the process.
Reports had been received from locals that the Japanese had brought reinforcements down from Soraken, Taiof Island and Tarlena, and at this point the Australians realised that the Japanese were concentrated in far greater numbers than anticipated and had in fact established their line of resistance in the sector upon the ridge. This was confirmed later after the Australians moved forward once more on 21 January, moving to within of the Genga River, where the track upon which they were advancing opened into a large garden that curved around the edge of Tsimba Ridge, from where the Japanese defenders had an excellent line of sight. Subsequent patrols determined the extent of the Japanese position, reporting that they had constructed a well-fortified position with a continuous trench along the crest of ridge, weapons pits and pill boxes, all with clear fields of fire, stretching up to .
The Australian commander, Kelly, ordered 'B' Company to move up and take the ridge and over the course of a week between 21 and 28 January they made several attempts against various parts of the ridge, approaching from the south, southeast and the north. These proved fruitless, although one platoon managed to encircle the position from the north before being engaged with heavy machine gun fire. On 23 January, the Australians established fire support in the form of guns from the 2nd Mountain Battery. Another three unsuccessful attempts were made on 24 January, and on 25 January 'C' Company carried out a movement to the flank in an attempt to get around behind the ridge and attack from the north. Crossing the crocodile-infested waters of the Genga about from the ridge, the Australians established a bridgehead across the river with a line of communications back to the Pimple, which they kept open by constantly patrolling along it.