The Battle of the Koromokina Lagoon was fought between the United States Marine Corps and Imperial Japanese Army forces on Bougainville Island.

In response to the Allied landings on Bougainville at Cape Torokina, Japanese General Hitoshi Imamura, commander of the 8th Area Army at Rabaul, determined to launch a counterattack.

Imamura intended to deploy about 3,000 troops from Rabaul; however, US air and naval activity prevented this and limited the Japanese to a smaller force.

Meanwhile, a force of 475 troops was dispatched from Rabaul to the Torokina area to conduct a counter-landing on the left flank in coordination with the assault on the right, while a further 700 were sent to reinforce the Japanese garrison at Buka.

As Japanese resistance to the landing petered out, Turnage began reorganizing his forces around the perimeter, establishing patrols and unloading supplies, and commencing important base development work.

[9] Losses during the naval battle of Empress Augusta Bay further delayed the sailing of the force, and another attempt was cancelled on 5 November after an air raid.

[11] Shortly after midnight, the transport group entered the objective area, but the first landing attempt was hurriedly abandoned when Allied ships were discovered blocking the way.

[11] They unloaded the troops about two miles out from the beaches in Atsinima Bay, with the landing forces going ashore in 21 ramped barges, cutters and motor boats.

The landing forces were put ashore near the Laruma River and Koromokina Lagoon, 2 to 3 miles northwest of the Marines perimeter, and were initially unopposed.

The Japanese, seeking cover from the artillery fire, had dug-in rapidly and, by taking advantage of abandoned foxholes and emplacements of the departed 1st and 2nd Battalions, 9th Marine Regiment, had established a hasty but effective defensive position.

[17] The two companies of the 1st Battalion, 3rd Marines—under the command of Major John P. Brody—found the going tough as the Japanese positions were well-hidden, and they found themselves under heavy machine gun and automatic weapons fire.

One of these, a platoon from Company K, 3rd Battalion, 9th Marine Regiment had scouted the upper Laruma River region, and ambushed a pursuing Japanese patrol several times before escaping into the interior.

The next morning, the attention of an Allied plane was attracted and within an hour the platoon was picked up by a tank lighter and returned to the main lines.

The other isolated unit—a patrol from Company B—was cut off from the rest of the battalion during the fighting and spent the night of 7–8 November behind Japanese lines without detection.

A hasty defense was mounted by rear echelon troops, subsequently turned back the assault, protecting the medical staff.

[23] On the morning of 9 November, the area between the Marine positions and the Laruma River was bombed and strafed by American dive bombers from Munda Airfield on New Georgia.

There was no further enemy activity on the left flank of the perimeter, and at noon of that day, control of the sector passed to the US 148th Infantry Regiment of the 37th Division, which had just arrived.

In addition, the Japanese had chosen to assault over such a large front that they had been unable to concentrate in sufficient numbers to take advantage of the initial surprise they had achieved.

With the beachhead secure and command having transitioned ashore,[29] US forces slowly pushed their perimeter forward, systematically advancing to several inland defense lines, resulting in further actions around Coconut Grove, Piva Forks and Hellzapoppin Ridge and Hill 600A.

As the conditions necessary for a large-scale counterattack faded, the Japanese 17th Army commander, Hyakutake, was ordered to delay his plans, postponing the assault on Torokina until March 1944.