Occurring between 18 and 25 November 1943 on Bougainville Island in the South Pacific, the battle involved troops from the United States Marine Corps and the United States Army fighting against Imperial Japanese Army forces and took place amidst the context of the expansion of a beachhead that US forces had established around Torokina on the western side of the island.

After the initial US attack was repulsed, the Japanese counterattacked before the US Marines overcame this and continued their advance towards two forks in the Piva River.

On 1 November 1943, the 3rd Marine Division, under Major General Allen H. Turnage, landed at Cape Torokina near Empress Augusta Bay on the western coast of Bougainville.

[1] The landings were undertaken as part of Allied efforts to isolate and reduce the main Japanese base around Rabaul under Operation Cartwheel.

General Roy Geiger, commander of I Marine Amphibious Corps,[6] set in place plans for the expansion of the beachhead perimeter that had been established around Torokina to a new defensive line further inland designated as "Easy".

After this, a roadblock and a defensive perimeter were established at the junction of the Numa–Numa Trail and the Piva River to defend against a potential Japanese counterattack.

The 3rd Battalion, 3rd Marine Regiment advanced towards the two forks of the Piva River to clear the threat, and two light tanks were disabled during the close fighting along the trail.

Moving forward under light opposition from scattered snipers and several machine gun nests, the enemy outposts were discovered to have been abandoned and had been booby-trapped.

[Note 2] A platoon—under the command of First Lieutenant Steve J. Cibik—was ordered to occupy the ridge with detachments of signals and a section of heavy machine guns.

[10] With daybreak on 21 November, it was discovered that the crest of the ridge was a Japanese outpost position, used during the day as an observation post and abandoned at night.

[15] The 3rd Battalion, 3rd Marine Regiment—having crossed the Piva River without trouble—advanced toward a slight rise, and as the lead scouts came over the top of this ridge, the Japanese opened fire from reverse slope positions.

Pinned down, the scouts held their positions while the rest of the battalion moved forward with a strong charge over the ridge and cleared the area of Japanese.

An attack was launched against the roadblock and managed to pass through the first line of bunkers after heavy fighting at close range but could make no further headway.

[17] As armor and equipment was moved into positions behind the 3rd Marine Regiment, the roads were extended by engineers and Seabees who advanced them as close to the river forks as possible.

Operating under fire from Japanese snipers and mortars, they erected bridges across the river,[17] and large quantities of supplies of ammunition, rations, and medical supplies were sent forward via a relay system to the front lines, while wounded were tended to at a medical station that had been set up close to the end of the road prior to evacuation.

[19] On 23 November, the 12th Marine Regiment's artillery observers moved to the crest of the ridge occupied by Cibik's platoon, in preparation for the attack the next day.

[20] Smoke shells were also fired into the hills east of the Torokina River to reduce the ability of the Japanese to observe the Marine positions.

[20] The enemy battery was located on the forward slope of a small coconut grove several thousand yards from the Piva River.

Japanese artillery burst along the line, traversing the front of the advancing Marines together with extremely accurate 90 mm mortar fire hit the attacking companies and caused significant casualties.

Without stopping, the 3rd Battalion, 3rd Marine Regiment went straight through the enemy flanking attempt and fought a violent hand-to-hand and tree-to-tree struggle that completely destroyed the Japanese force.

As this movement began again, the Japanese called in their own mortars to seek out the American crews; the infantry attack continued, while overhead supporting and defensive fire was exchanged.

Although the remaining Japanese made a determined final stand on the objective, the Marines were able to push ahead, and as the resistance subsided the battalion "mopped up" before establishing a defensive night position.

[23] Meanwhile, the US advance continued throughout the day until they met heavy Japanese resistance at a knoll dubbed "Grenade Hill", which was eventually captured by mid-morning on 26 November.

[26][27] By the conclusion of the Piva Forks fighting, US forces were in control of the majority of the key terrain and high features that existed west of the Torokina River.

These reduced the Japanese force's ability to harass the US beachhead around Torokina, and as a result although the fighting on Bougainville continued with notable actions being fought at Koiari and Hellzapoppin Ridge and Hill 600A, in the words of historian Samuel Morison, "the Battle of Piva Forks...was the last serious ground resistance for a long time".

[24] Indeed, it was not until March 1944 that the Japanese were able to make any real attempt at counterattacking when they launched a large-scale offensive on the American perimeter with 15,000 troops drawn from the 6th and 17th Divisions who attacked north and west from Buin and Numa–Numa.