Battle of Koromokina Lagoon

The Battle of the Koromokina Lagoon was fought between the United States Marine Corps and Imperial Japanese Army forces on Bougainville Island. It took place on 7–8 November 1943 during the Bougainville campaign.

Background
Responding to the Allied landings on Bougainville at Cape Torokina on 1 November 1943, Japanese General Hitoshi Imamura—commander of the 8th Area Army at Rabaul—decided to send a force to counter the Allied landing.

A force of 850 soldiers was organised around several regiments of the Japanese Army's 17th Division, which included the 5th Company, 54th Infantry Regiment, the 6th Company, 53rd Infantry Regiment, a platoon from the 7th Company, 54th Infantry Regiment and a machine gun company from the 54th Infantry, with some attached service troops.

The force sailed initially on the night of 1 November; however, due to Japanese losses in the naval battle of Empress Augusta Bay, it convinced them that a counterlanding at this time would be difficult, with the attempt postponed and the troops returned to Rabaul. Again, the sailing of the force was cancelled on 5 November after an air raid. The landing force finally departed Rabaul on 6 November, the troop destroyers JAPANESE DESTROYER Amagiri, JAPANESE DESTROYER Uzuki, JAPANESE DESTROYER Yūnagi and JAPANESE DESTROYER Fumizuki were screened by the cruisers JAPANESE CRUISER Agano and JAPANESE CRUISER Noshiro and the destroyers JAPANESE DESTROYER Urakaze, JAPANESE DESTROYER Kazagumo, JAPANESE DESTROYER Wakatsuki, JAPANESE DESTROYER Makinami, JAPANESE DESTROYER Naganami, JAPANESE DESTROYER Ōnami and JAPANESE DESTROYER Hayanami.

Opening moves
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. The destroyers headed north again and back-tracked closer to the shoreline for a second try. 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 and was initially unopposed. The landing had required a bombardment from the naval force; however, due to an Allied fleet nearby, this was not carried out.

Detected by the US naval forces after a Japanese barge was sighted about 4 mi north of Cape Torokina, a PT boat was assigned to investigate; however, reports from US 3rd Marine Division troops on that flank of the beachhead confirmed the fact that enemy barges were landing troops at scattered points along the shoreline and that the Marines were now engaging them. Artillery fire from the 12th Marine Regiment, coastal defense guns and 90 mm anti-aircraft batteries of the 3rd Defense Battalion opened fire on the enemy barges and landing beaches.

The Japanese landings had scattered over a wide area, typical of landings in darkness and rough surf. Unable to reassemble quickly, the Japanese were faced with attacking with small units until their forces could combine. The first attack was carried out by less than 100 Japanese soldiers.

The 3rd Battalion, 9th Marine Regiment—under the command of Lieutenant Colonel Walter Asmuth, Jr.—was tasked with stopping the enemy counterthrust. Artillery support fire was placed in front of the perimeter and along the beach. At 08:20, Company K, 3rd Battalion—with a platoon from regimental weapons company attached—moved forward to blunt the Japanese counterattack. About 150 yd from the main line of resistance, the advancing Marines hit the front of the enemy force. 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.

Heavy fighting broke out with the Japanese firing light machine guns from well-concealed fortifications covered by automatic rifle fire from tree snipers. Company K's attack stalled and was pinned down; their advance was halted. Japanese resistance increased as reinforcements from the remainder of the counterlanding force began to arrive. At 13:15, the 1st Battalion, 3rd Marine Regiment who were in reserve, was ordered into the battle.

Company K provided covering fire while Company B of 1st Battalion, 3rd Marine Regiment moved across the left flank and passed through Company K to take up the fight. Company C of 1st Battalion, 3rd Marine Regiment moved forward on the right flank and Company K withdrew, having lost five killed and 13 wounded, two of whom later died.

Tank support
The two companies of 1st Battalion, 3rd Marine Regiment—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. Tanks moved up to help with the assault, and the Marine advance inched along as the tank fire eliminated the enemy emplacements. Late that afternoon, the advance was halted and a heavy artillery concentration, in preparation for a full-scale attack by the 1st Battalion, 21st Marine Regiment against the enemy defenses in front of the Marines. The artillery fire raged through the enemy positions and Companies B and C placed mortar fire almost on top of their own positions.

The attack by 1st Battalion, 21st Marine Regiment—under the command of Lt. Col. Ernest W. Fry, Jr.—was set for 17:00, but it was postponed until the next morning (8 November). Several Marine units had been cut off from the main forces during the day, with a platoon from Company K, 3rd Battalion, 9th Marine Regiment having scouted the upper Laruma River region, ambushed a pursuing Japanese patrol several times before escaping into the interior. The platoon returned to the Marine lines 30 hours later, with one man wounded and one man missing after inflicting a number of casualties on the enemy landing force. Another patrol from Company M, 3rd Battalion, 9th Marine Regiment was cut off on the beach between two enemy forces and when the radio of the artillery officer with the patrol did not function, he headed back to the main lines directing an artillery barrage that landed on the Japanese position to the left of the patrol from Company M. The patrol then moved toward the Marine lines, only to find the beach blocked by enemy forces opposing Company K. A message was written in the sand of the beach, which was seen by an air spotter with two tank lighters evacuating the patrol from the beach. Sixty men were evacuated successfully 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 from 1st Battalion, 3rd Marine Regiment—scouting the enemy's flank position—slipped through the jungle and passed by the enemy force without being observed. Choosing to head for the beach instead of the interior, the platoon struggled to the coast. There, the patrol cleaned its weapons with gasoline from a wrecked barge, and spent the night in the jungle. 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 the enemy's lines without detection.

After a 20-minute preparatory barrage by five batteries of artillery together with machine gun, mortar and antitank-gun fire on the morning of 8 November, the 1st Battalion, 21st Marine Regiment passed through the lines of the 1st Battalion, 3rd Marine Regiment companies and began the attack. The infantry were supported by light tanks. Only a few Japanese soldiers fought off the attack with these being killed or captured. More than 250 dead Japanese were found in the area. Moving about 1500 yd through the jungle parallel to the shoreline, no further opposition was encountered. That afternoon, a defensive line was established behind a lagoon and extensive patrols were sent out without making any enemy contact.

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. Patrols found the bodies of many Japanese, who had taken refuge in the Laruma River area. 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. The battalion from the 9th Marines moved to the right flank, and 1st Battalion, 3rd Marine Regiment withdrew to the 3rd Marine Regiment area. The 1st Battalion, 21st Marine Regiment held the left-flank and remained under operational control of the 148th Regiment until other units of the 37th Division arrived.

Aftermath
The Japanese attempt to destroy the Allied landing force by counter-landing had failed, due to underestimating the size of the Allied landing forces. Out of a force of 850 soldiers which were sent, only 475 were landed. Over 250 were killed, the rest withdrew into the jungle. Most of those were killed in the artillery barrages and the air strike between 7 and 9 November. The landing site was also an unfortunate choice: its location was very close to that of the Allied beachhead, which the Japanese believed to be further east around Cape Torokina.

The Marines lost 17 killed and 30 wounded.