[4] 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After an initial counter-attack that was repulsed, the marines had been replaced by a garrison of US Army troops who began consolidating their position around Torokina, establishing a strong perimeter.

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.

[10] 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,[11] as the Australians attempted to drive the Japanese in the sector on to the narrow Bonis Peninsula.

[13] 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",[14] it was a "textbook fortress".

[15] According to Aldridge, it was a "maze of dugouts, trenches, [and] foxholes",[14] 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.

[11] The 31st/51st Infantry Battalion, under the command of Lieutenant Colonel Joseph Kelly[17][Note 2] 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.

[11][19] 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.

[11][20] 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,[Note 3] firing for the first time in the sector.

[23] 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.

Crossing the crocodile-infested waters of the Genga about 600 yards (550 m) 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.

The Australian counter-attack, organised by the 'C' Company's commander, Captain Alwyn Shilton, and supported by accurate artillery fire, came on 1 February and resulted in over 30 Japanese killed or wounded.

Supported by a barrage of over 500 artillery shells and mortar bombs, and airstrikes from Australian Wirraways and New Zealand Corsairs,[29] three platoons from Captain Millett Harris's 'B' Company assaulted the ridge from different points in an effort to encircle the defenders.

[31] Throughout the night, Japanese artillery was very active, firing down on the Australian positions around The Pimple, after which a "banzai attack" was launched early on 7 February.

[33][32] One of the Australians, Private Colin Jorgensen, was awarded the Military Medal for charging a Japanese weapon pit and knocking it out of action during the final assault.

They had been prepared to hold the position, and had the numbers to do it; however, Nakamura had decided to withdraw from the ridge when the morale of his troops had fallen due to lack of ammunition and food, and rising casualties from tropical diseases.

[36] The real size of the force holding Tsimba Ridge was also revealed by documents captured after the war, highlighting the gross underestimation of Japanese strength by Australian intelligence sources.

[38][39] The 31st/51st would later take part in the failed landing at Porton Plantation in June, where they would suffer heavy casualties, including Downs, who was killed leading his company.