The 'Battle of the Koromokina Lagoon' was fought between Japanese and US forces on Bougainville island in the aftermath of the 'Cherryblossom' landing by elements of the US Marine Corps (7/8 November 1943).

In response to the landing at Cape Torokina, General Hitoshi Imamura, commander of the 8^th Area Army headquartered at Rabaul on New Britain island, decided to launch a counterattack. Wholly underestimating the size of the US landing force, the Japanese despatched a force of just 850 men to undertake a counter-landing and drive the US forces back into the sea. The Japanese soldiers landed from four destroyers near Koromokina Lagoon on the night of 7 November and engaged two battalions of the US Marine Corps from the 3^rd and 9^th Marine Regiments under the command of Major General Allen H. Turnage. Over the next two days the Japanese attacks were defeated with heavy losses. After the battle, US forces continued to expand their Empress Augusta Bay beach-head with the object of constructing airfields to attack and neutralise the Japanese forces based at Rabaul and nearby areas.

Responding to the Allied landings to each side of Cape Torokina in Empress Augusta Bay on 1 November, Imamura decided to send a force to counter the Allied landing, initially intending to despatch about 3,000 men from Rabaul. US naval and air activity prevented this, however, and limited the Japanese to a the commitment of a somewhat smaller force. The Japanese commander on Bougainville, Lieutenant General Harukichi Hyakutake, commander of the 17^th Army, despatched a large group of reinforcements from the 23^rd Regiment from the main Japanese position around Buin, on the southern tip of Bougainville, to assault the right flank of the US beach-head. Meanwhile, some 475 men were despatched from Rabaul to the Torokina area to make a counter-landing on the left flank in co-ordination with the assault on the right, while a further 700 men were sent to reinforce the Japanese garrison on Buka, the small island just to the north of Bougainville. These troops were drawn from Lieutenant General Yasushi Sakai’s 17^th Division, recently redeployed to the theatre from China, and included several companies of the 53^rd Regiment and 54^th Regiment. With the addition of support and service troops, the Japanese landing force totalled some 850 men.

The Japanese had estimated US strength in the area as something in the order of 5,000 to 10,000 men, whereas the reality was more than 14,000 men landed around Cape Torokina on the first day of 'Cherryblossom'. These were followed by another 3,500 men on 6 November. These troops were of Turnage’s 3^rd Marine Division, which had landed as part of Rear Admiral Theodore S. Wilkinson’s 3^rd Fleet Amphibious Forces. After the initial landing Turnage, assisted by Brigadier General Oscar R. Cauldwell, had been in tactical command of the US troops ashore while Lieutenant General Alexander A. Vandegrift returned to Guadalcanal with Wilkinson. As Japanese resistance to the landing petered out, Turnage began to reorganise his force around the perimeter of the beach-head, establishing patrols and unloading supplies, and embarking on the important base development work that was the object of the whole US undertaking.

The Japanese force sailed initially on the night of 1/2 November, but was sighted by US aircraft, whose sighting report triggered the despatch of a large US force to attack it. This convinced the Japanese that a counter-landing at this time would be difficult, so the attempt postponed and the troops returned to Rabaul. 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. The landing force finally departed Rabaul on 6 November aboard four destroyers. These were covered by a escort under the command of Rear Admiral Morikazu Osugi. This escort included the light cruiser Agano, and the destroyers Naganami, Wakatsuki, Shigure, Samidare, Shiratsuyu and Amagiri.

Shortly after 00.00, the transport group entered the area in which the landing was to be made, but the first attempt to do so was hurriedly abandoned when Allied ships were discovered to be blocking the way. The destroyers headed to the north again and back-tracked closer to the coast for a second attempt. A small US naval force of eight PT-boats had been established around Puruata island, but none of these boats detected the Japanese force. The destroyers unloaded the troops about 2 miles (3.2 km) out from the beaches in Atsinima Bay to complete their passage in 21 ramped barges, cutters and motor boats. The landing force was put ashore near the Laruma river and the Koromokina Lagoon, and were initially unopposed. As planned, the landing was to have been supported by a bombardment from the naval force, but as a result of the presence in the area of a US naval force, this was not carried out. After the landing, the Japanese destroyers withdrew.

The landings initially confused the defending marines, who believed that the landing craft were American, and this delayed the US response. Meanwhile, Japanese troops cut off a marine outpost, which was withdrawn during the course of the following day by naval landing craft, and occupied several defensive positions that had been abandoned by the marines on the previous day during the reorganisation. After a Japanese barge was sighted about 4 miles (6.4 km) to the north of Cape Torokina, a PT-boat was assigned to investigate, but reports from 3^rd Marine Division troops on that flank had already confirmed the fact that Japanese barges were landing troops at scattered points along the coast and that the marines were now engaging them. Artillery fire from the 12^th Marine Regiment, coastal defence guns and 90-mm (3.54-in) anti-aircraft batteries of Lieutenant Colonel Edward H. Forney’s 3^rd Defense Battalion opened fire on the Japanese barges and landing beaches. The Japanese landings had been scattered over a wide area, as night landings often were, and by rough surf. Unable to assemble quickly, the Japanese were initially forced to attack in small numbers: the first attack was carried out by fewer than 100 men.

The 3/9^th Marine Regiment, under the command of Lieutenant Colonel Walter Asmuth, was tasked with stopping the Japanese counterthrust. Artillery support fire was called down along the front of the perimeter and the beach. At 08.20, Company K of the 3^rd Battalion, with a platoon from the regimental weapons company in support, set out to tackle the Japanese counterattack. About 150 yards (140 m) from the main line of resistance, the advancing marines hit the front of the Japanese force. Seeking cover from the artillery fire, the Japanese had dug-in rapidly and, by taking advantage of the abandoned foxholes and emplacements of the departed 1 and 2/9^th Marines, had hastily established an effective defensive position.

Heavy fighting broke out with the Japanese firing light machine guns from well-concealed positions covered by the rifle fire of snipers hidden in trees. Company K’s attack stalled and was thus pinned down and brought to a halt. Japanese resistance increased as reinforcements from the remainder of the counter-landing force began to arrive. At 13.15, the 1/3^rd Marines, in reserve, was ordered into the battle after transfer from the right flank of the US beach-head. Company K provided covering fire while Company B of the 1/3^rd Marines moved across the left flank and passed through Company K to take up the fight. Company C of the 1/3^rd Marines moved forward on the right flank and Company K withdrew, having lost five men killed and 13 wounded, of whom two later died.

The two companies of the 1/3^rd Marines, under the command of Major John P. Brody, found the going difficult as the Japanese positions were well-hidden, and the marines found themselves under heavy machine gun and automatic weapons fire. Tanks moved up to help with the assault, and the marines advanced slowly as the tank fire eliminated the Japanese emplacements. Late in the afternoon of this day, the US advance was halted and a heavy artillery concentration, in preparation for a full-scale attack by the 1/21^st Marines against the Japanese positions. The artillery fire saturated the Japanese positions and Companies B and C placed mortar fire almost on top of their own positions.

The attack by the 1/21^st Marines, under the command of Lieutenant Colonel Ernest W. Fry, was scheduled for 17.00, but was postponed until the morning of 8 November. Throughout the night, groups of Japanese troops pushed through US lines, threatening the field hospital established by the 3^rd Medical Battalion. A hasty defence, mounted by rear-echelon troops, subsequently turned back the assault and thus protected the medical facility and its personnel.

Several marine units had been cut off from the main force during 7 November. One of these, a platoon of Company K, 3/9^th Marines, had scouted the upper Laruma river region, and ambushed a pursuing Japanese patrol several times before escaping into the interior. The platoon returned to the US lines 30 hours later, with one man wounded and one man missing, after inflicting a number of casualties on the Japanese landing force. Another patrol, of the 3/9^th Marines' Company M, was cut off on the beach between two Japanese forces, and when the radio of the artillery officer with the patrol failed, he headed back to the main lines to direct an artillery barrage which landed on the Japanese position to the left of Company M’s patrol. The patrol then moved toward the marine lines only to find the beach blocked by Japanese forces opposing Company K. A message written in the beach’s sand seen by an air spotter, and two tank lighters evacuated the patrol from the beach. Some 60 men were evacuated after killing an estimated 35 Japanese; only two of the marines had been wounded.

Two other Marine groups became isolated in the fighting along the perimeter. One platoon of the 1/3^rd Marines, which was scouting the Japanese flank position,slipped through the jungle and passed by the Japanese without being detected. Choosing to head for the beach instead of the interior, the platoon then struggled to the coast, where the men cleaned their weapons with fuel from a wrecked barge, and spent the night in the jungle. During the morning of the following day, the attention of a US aeroplane was attracted and within an hour the platoon had been picked up by a tank lighter and returned to the main lines. The other isolated unit was a patrol of Company B, and this was cut off from the rest of the battalion during the fighting. The patrol spent the night of 7/8 November behind the Japanese lines without being detected.

After a 20-minute preparatory barrage by five batteries of artillery together with the fire of machine guns, mortars and anti-tank guns in the morning of 8 November, the 1/21^st Marines passed through the lines of the 1/3^rd Marine companies and began the attack with the support of light tanks. Only a few Japanese soldiers tried to fight off the attack, these being killed or captured. More than 250 dead Japanese were found in the area. Moving about 1,500 yards (1370 m) through the jungle parallel to the shoreline, the marines met no further opposition. That afternoon, a defensive line was established behind a lagoon and extensive patrols were sent out without making contact with the Japanese.

On the morning of 9 November, the area between the marine positions and the Laruma river was bombed and strafed by US dive-bombers from Munda airfield on New Georgia island. Patrols found the bodies of many more Japanese who had taken refuge in the Laruma river area. There was no further Japanese activity on the left flank of the perimeter, and at 12.00 on this day, control of the sector passed to the US Army’s 148^th Infantry of Major General Robert S. Beightler’s 37^th Division, which had just arrived. The battalion of the 9^th Marines moved to the right flank, and the 1/3^rd Marines withdrew to the 3^rd Marines' area. The 1/21^st Marines held the left flank and remained under operational control of the 148^th Infantry until other units of the 37^th Division arrived.

The Japanese attempt to destroy the US landing force had failed as a result of underestimating the size of the US landing forces and also the impossibility of co-ordinating actions on both sides of the perimeter. Of a force of 850 men, only 475 actually landed. At least 377 men were killed, and the survivors withdrew into the jungle. Most of these were killed in artillery barrages and air attacks between 7 and 9 November. For the Japanese, the landing site was also an unfortunate choice: its location was very close to that of the US beach-head, which the Japanese believed to be farther to the east around Cape Torokina. In addition, the Japanese had chosen to attack across so large a front that they had been unable to create a concentration of force sufficient to take advantage of the initial surprise they had achieved. The Marines lost 17 men killed and 30 wounded.

On 9 November, Major General Roy S. Geiger assumed command of I Marine Amphibious Corps in succession to Vandegrift. Four days later, Geiger assumed command of all Allied forces on Bougainville and in the immediate vicinity. After the fighting around the Koromokina Lagoon, the 23^rd Regiment began to arrived from the south, and subsequently launched an attack on a US force holding a roadblock around the junction of the Piva and Numa Numa Trails, which were important avenues of approach towards the Allied beach-head. The subsequent Battle for the Piva Trail resulted in heavy Japanese casualties, with around 550 men killed. After this, US Army elements began arriving in force to reinforce the marines as further echelons arrived at Cape Torokina, bringing more supplies and expanding the size of the US force on Bougainville still further. Another 5,715 men had landed on 8 November, followed by 3,599 and 6,678 more on 11 and 13 November respectively. With the beach-head secure and expanded into the beginnings of a lodgement, and command transitioned ashore, US forces slowly pushed their perimeter forward, systematically advancing to several inland defence lines, resulting in further actions around Coconut Grove, Piva Forks, Hellzapoppin Ridge and Hill 600A.

Initially, Japanese commanders had believed that the landing at Cape Torokina was only temporary and would be followed up by another move on Buka. As a result, they held off launching a concerted counterattack even though there were up to 15,000 troops available in southern Bougainville. Meanwhile, further base development saw the US forces establish several airfields inside the Torokina perimeter, and these were used to project air power toward the main Japanese base at Rabaul, while Japanese sea lanes of communication between Rabaul and Bougainville were heavily interdicted. As the conditions necessary for a large-scale counterattack faded, Hyakutake, commander of the 17^th Army, was ordered to delay his plans, postponing the assault on the Torokina lodgement until March 1944.