The attack was hampered by inaccurate intelligence and poor planning and was pushed back by the well-prepared Allied defenders, who greatly outnumbered the Japanese force.

Following the engagement the Japanese force withdrew from the Empress Augusta Bay area, and only limited fighting took place until late 1944, when Australian troops took over from the Americans and began a series of advances across the island until the end of the war in August 1945.

[11] The Bougainville campaign began on 1 November 1943 when the United States I Marine Amphibious Corps landed at Cape Torokina, around Empress Augusta Bay on the west coast of the Japanese-held island.

[12][13] The Allied invasion aimed to establish a defensive perimeter around Cape Torokina within which airfields would be built to attack the major Japanese base at Rabaul and support other operations in the region.

The planners judged that any force large enough to pose a threat to the beachhead would need at least three months to reach the area by travelling overland from the Japanese bases.

While a Japanese counter-landing could potentially deposit a large force in the Cape Torokina area, the United States Navy was confident that it could stop any such operation.

[15] The Japanese commander on Bougainville, Lieutenant General Harukichi Hyakutake, initially believed that the landing at Empress Augusta Bay was a diversion and would be followed by a direct assault on the south of the island.

[18] After pushing the Japanese back, the American forces began work on building defense lines to protect the airfield complex in late November.

Artillery and mortars were emplaced in positions where they could support any part of the defensive perimeter, and fire plans were developed to allow for rapid bombardments of all possible approach routes.

[25] In late December 1943 Hyakutake and the other senior Japanese officers on Bougainville concluded that the Allies did not intend to advance from their perimeter at Empress Augusta Bay or land elsewhere on the island, and they began planning a counterattack.

[27] During a visit to Bougainville on 21 January 1944, General Hitoshi Imamura, the commander of the Eighth Area Army, directed that the offensive should be launched in early March.

As the bulk of the 17th Army was stationed in northern and southern Bougainville, engineers needed to develop roads and bridges to allow the troops to move to the hills inland from the American perimeter.

[21] The combat troops chosen for the attack spent 40 days throughout January and into February conducting unit level training, perfecting jungle assault techniques.

Barges moved artillery, other equipment and 1,400 soldiers to a point east of Cape Torokina, around the Jaba–Mosigeta area;[29][30] the guns and supplies were then carried overland into the hills.

[21] PT boats and Landing Craft Infantry gunboats supported by PBY Catalina aircraft patrolled the coast of Bougainville and attacked Japanese barges but were unable to stop the movement of supplies and equipment by sea.

The Fijian force at Ibu was reinforced to a strength of about 400 personnel on 3 February but was withdrawn to the perimeter in the middle of the month after a larger body of Japanese troops surrounded the outpost and began to attack the U.S. Army units which were protecting its supply lines.

Intelligence information detailing the Japanese plans was then distributed to American soldiers holding the perimeter through various means, including notices posted on unit bulletin boards.

[14][31] According to U.S. Army official historian John Miller, the total number of men in the attacking force was either 15,400 or 19,000,[35] although Shindo states that only 9,548 were directly committed to the fighting.

Each day, American SBD and TBF aircraft flew more than 100 sorties over Bougainville in direct support of the ground troops, and USMC and RNZAF fighter bombers attacked Japanese supply lines.

While this effort was initially successful and led to the extraction of several American soldiers who had been isolated on the position, by late afternoon the U.S. units had been forced to withdraw to the North Knob.

As casualties mounted, McCulloch resolved to abandon the direct approach, instead deciding that he would seek to wear down the defenders on the South Knob after patrols discovered that the Japanese had no reserves left to reinforce the position.

[72][73] The Magata Unit approached Cape Torokina from the north by moving along the Logging Trail which had been built by U.S. engineers and entered the northwestern sector of the perimeter near Taylor's Creek.

In the daylight on 24 March, U.S. troops launched a strong counterattack, supported by tanks and seven artillery battalions,[79] after large numbers of reserves were poured in to the sector.

Air attacks fell on the Iwasa Unit's rear area and amidst intense fighting, a battalion of the Japanese 45th Infantry Regiment was completely destroyed, while another from the 53rd almost suffered the same fate.

As it was believed that the Japanese would attempt to use barges to land a force inside the perimeter, the destroyers and PT boats stationed at Bougainville patrolled along the coast of Empress Augusta Bay each night.

As they began to move, elements of the Japanese 6th Cavalry Regiment[Note 1] and the 2nd Battalion, 4th South Seas Garrison Unit, acted as a screening force to cover their movement.

Even if XIV Corps had been as weak as the Japanese believed, the force committed to the attack did not include enough troops or artillery to penetrate the well-prepared defenses.

[87] Miller opines that had the offensive been successful, it would have had a serious effect on the campaign in the Solomons, resulting in a large drain on Allied resources, but would most likely not have altered the wider course of the war.

Well-stocked with supplies, equipment and amenities, including medical and recreation facilities, it became a symbol of Allied power and wealth that was used to impress the local Bougainvilleans.

This situation began to change in late 1944, when the Australian II Corps, under the command of Lieutenant General Stanley Savige, started to relieve the U.S. forces,[91] who were transferred to the Philippines.