The Battle of the Tenaru, sometimes called the Battle of the Ilu River or the Battle of Alligator Creek, was a land battle between the Imperial Japanese Army and Allied ground forces that took place on 21 August 1942, on the island of Guadalcanal during the Pacific campaign of World War II. The battle was the first major Japanese land offensive during the Guadalcanal campaign.
In the battle, U.S. Marines, under the overall command of U.S. Major General Alexander Vandegrift, repulsed an assault by the "First Element" of the "Ichiki" Regiment, under the command of Japanese Colonel Kiyonao Ichiki. The Marines were defending the Lunga perimeter that guarded Henderson Field which had been captured by the Allies in landings on Guadalcanal on 7 August. Ichiki's unit was sent to Guadalcanal in response to the Allied landings there with the mission of recapturing the airfield and driving the Allied forces off the island.
Underestimating the strength of Allied forces on Guadalcanal, which at the time numbered about 11,000 personnel, Ichiki's 900-man unit conducted a nighttime frontal assault on Marine positions at Alligator Creek on the east side of the Lunga perimeter. Jacob Vouza, a Coastwatcher scout, warned the Americans of the impending attack minutes before Ichiki's assault. The Japanese were defeated with heavy losses. The Marines counterattacked Ichiki's surviving troops after daybreak, killing many more. About 800 of the original 917 of the Ichiki Regiment's First Element died.
The battle was the first of three separate major land offensives by the Japanese in the Guadalcanal campaign. The Japanese realized after Tenaru that Allied forces on Guadalcanal were much greater in number than originally estimated and subsequently sent larger forces to the island in their attempts to retake Henderson Field.
Background
During the Pacific campaign of World War II, on 7 August 1942, U.S. forces landed on Guadalcanal, Tulagi, and Florida Islands in the Solomon Islands. The landings on the islands were meant to deny their use by the Japanese as bases for threatening the supply routes between the U.S. and Australia, and to secure the islands as starting points for a campaign with the eventual goal of isolating the major Japanese base at Rabaul while also supporting the Allied New Guinea campaign. The landings initiated the six-month-long Guadalcanal campaign.
Taking the Japanese by surprise, the Allied landing forces accomplished their initial objectives of securing Tulagi and nearby small islands, as well as an airfield then under construction at Lunga Point on Guadalcanal, by nightfall on 8 August. That night, as the transports unloaded, the Allied warships screening the transports were surprised and defeated by an Imperial fleet of seven cruisers and one destroyer, commanded by Japanese Vice Admiral Gunichi Mikawa. One Australian and three U.S. cruisers were sunk and one other U.S. cruiser and two destroyers were damaged in the Battle of Savo Island. Rear Admiral Richmond K. Turner withdrew all remaining Allied naval forces by the evening of 9 August without unloading all the heavy equipment, provisions, and troops from the transports, although most of the divisional artillery was landed, comprising thirty-two 75 mm and 105 mm howitzers. Only five days' rations were landed.
The Marines ashore on Guadalcanal initially concentrated on forming a defense perimeter around the airfield, moving the landed supplies within the perimeter, and completing construction of the airfield. Major General Alexander Vandegrift placed his 11,000 troops on Guadalcanal in a loose perimeter around the Lunga Point area. In four days of intense effort, the supplies were moved from the landing beach into dispersed dumps within the perimeter. Work began on completing the airfield immediately, mainly using captured Japanese gear. On 12 August, the airfield was named Henderson Field after Major Lofton Henderson, a Marine aviator who had been killed at the Battle of Midway. Captured Japanese stock increased the total supply of food to 14 days' worth. To conserve the limited food supplies, the Allied troops were limited to two meals per day.
In response to the Allied landings on Guadalcanal, the Japanese Imperial General Headquarters assigned the Imperial Japanese Army's 17th Army, a corps-sized command based at Rabaul and under the command of Lieutenant-General Harukichi Hyakutake, with the task of retaking Guadalcanal from Allied forces. The 17th Army, then heavily involved with the Japanese campaign in New Guinea, had only a few units available to send to the southern Solomons area. Of these units, the 35th Infantry Brigade under Major General Kiyotake Kawaguchi was at Palau, the 4th (Aoba) Infantry Regiment under Major General Yumio Nasu was in the Philippines, and the 28th (Ichiki) Infantry Regiment, under the command of Colonel Kiyonao Ichiki, was at sea en route to Japan from Guam. The different units began to move towards Guadalcanal immediately, but Ichiki's regiment, being the closest, arrived first.
An aerial reconnaissance of the U.S. Marine positions on Guadalcanal on 12 August by one of the senior Japanese staff officers from Rabaul sighted few U.S. troops in the open and no large ships in the waters nearby, convincing Imperial Headquarters that the Allies had withdrawn the majority of their troops. In fact, none of the Allied troops had been withdrawn. Hyakutake issued orders for an advance unit of 900 troops from Ichiki's regiment to be landed on Guadalcanal by fast warship to immediately attack the Allied position and reoccupy the airfield area at Lunga Point. The remaining personnel in Ichiki's regiment would be delivered to Guadalcanal by slower transport later. At the major Japanese naval base at Truk, which was the staging point for delivery of Ichiki's regiment to Guadalcanal, Colonel Ichiki was briefed that 2,000–10,000 U.S. troops were holding the Guadalcanal beachhead and that he should "avoid frontal attacks."
Ichiki, together with 916 of his regiment's 2,300 troops, designated the "First Element" and carrying seven days' supply of food, were delivered to Taivu Point, about 35 kilometers (22 mi) east of Lunga Point, by six destroyers at 01:00 on 19 August. Ichiki was ordered to scout the American positions and wait for the remainder of his force to arrive. Known as the Ichiki Butai (Ichiki Detachment), they were an elite and battle-seasoned force but as was about to be discovered, they were heavily stricken with "victory disease" – overconfidence due to previous success. Ichiki was so confident in the superiority of his men that he decided to destroy the American defenders before the remaining majority of his force arrived, even writing in his journal "18 August, landing; 20 August, march by night and battle; 21 August, enjoyment of the fruit of victory." He concocted a brazenly simple plan: march straight down the beach and through the American defenses. Leaving about 100 personnel behind as a rear guard, Ichiki marched west with the remaining 800 men of his unit and made camp before dawn about 14 kilometers (8.7 mi) east of the Lunga perimeter. The U.S. Marines at Lunga Point received intelligence that a Japanese landing had occurred and took steps to find out exactly what was happening.
Disposition and constituents of the Ichiki detachment are as follows;
- Detachment Headquarters - 164 men
- Battalion HQ (2nd Battalion, 28th infantry) - 23 men
- 1st, 2nd, 3rd and 4th companies - 420 men (105 men each)
- Machinegun company (8 heavy machineguns) - 110 men
- Battalion Heavy Gun Platoon - 50 men, two 70mm battalion guns
- Engineer Company (1st Co., 7th Engineer Construction Regiment) - 150 men
Prelude
Reports from patrols of Solomon Islanders, including retired Sergeant Major Jacob C. Vouza of the British Solomon Islands Protectorate Constabulary, under the direction of Martin Clemens, a coastwatcher and officer in the British Solomon Islands Protectorate Defence Force (BSIPDF), along with Allied intelligence from other sources, indicated that Japanese troops were present east of Lunga Point. To investigate further, on 19 August, a Marine patrol of 60 men and four native scouts, commanded by U.S. Marine Captain Charles H. Brush, marched east from the Lunga perimeter.
Shibuya's Patrol
At the same time, Ichiki sent forward his own patrol of 38 men, led by his communications officer, Captain Yoshimi Shibuya, to reconnoiter Allied troop dispositions and establish a forward communications base. Around 12:00 on 19 August at Koli Point, Brush's patrol sighted and ambushed the Japanese patrol, killing all but five of its members, who escaped back to Taivu. The Marines suffered three dead and three wounded. Brush, a Reservist, was considered a capable leader and was later awarded a Silver Star for his actions.
Papers discovered on the bodies of some of the Japanese officers in the patrol revealed that they belonged to a much larger unit and showed detailed intelligence of U.S. Marine positions around Lunga Point. The papers did not detail exactly how large the Japanese force was or whether an attack was imminent.
Now anticipating an attack from the east, the U.S. Marine forces, under the direction of Vandegrift, prepared their defenses on the east side of the Lunga perimeter. Several official U.S. military histories identify the location of the eastern defenses of the Lunga perimeter as emplaced on the Tenaru River. The Tenaru River, however, was located further to the east. The river forming the eastern boundary of the Lunga perimeter was the Ilu River, nicknamed Alligator Creek by the Marines, a double misnomer: there are crocodiles not alligators in the Solomons, and Alligator "Creek" was a tidal lagoon separated from the ocean by a sandbar about 7 to 15 meters (23 to 49 ft) wide and 30 meters (98 ft) long.
Along the west side of Alligator Creek, Colonel Clifton B. Cates, commander of the 1st Marine Regiment, deployed his 1st (Lieutenant Colonel Leonard B. Cresswell) and 2nd Battalions (Lieutenant Colonel Edwin A. Pollock). To help further defend the Alligator Creek sandbar, Cates deployed 100 men from the 1st Special Weapons Battalion with two 37 mm anti-tank guns equipped with canister shot. Marine divisional artillery, consisting of both 75 mm and 105 mm guns, pre-targeted locations on the east side and sandbar areas of Alligator Creek, and forward artillery observers emplaced themselves in the forward Marine positions. The Marines worked all day on 20 August to prepare their defenses as much as possible before nightfall.
Learning of the annihilation of Shibuya's patrol, Ichiki quickly sent forward a company to bury the bodies and followed with the rest of his troops, marching throughout the night of 19 August and finally halting at 04:30 on 20 August within a few miles of the U.S. Marine positions on the east side of Lunga Point. At this location, he prepared his troops to attack the Allied positions that night.
Battle
Just after midnight on 21 August, Ichiki's main body of troops arrived at the east bank of Alligator Creek and were surprised to encounter the Marine positions, not having expected to find U.S. forces so far from the airfield. Nearby U.S. Marine listening posts heard "clanking" sounds, human voices, and other noises before withdrawing to the west bank of the creek. Ichiki consulted Major Kuramoto, the Battalion commander as well as the leading company commander. Ichiki then ordered an assault across the sandbar by a detachment of men, while the other troops provided covering fire. The green flare signaling the attack was fired at 01:30 and Ichiki's force opened fire with machine guns and mortars on the Marine positions on the west bank of the creek, and a first wave of about 100 Imperial soldiers from the 2nd company charged across the sandbar towards the Marines; crossing the sandbar would become the primary objective of frequent Japanese attacks.
Marine machine gun fire and canister rounds from the 37 mm cannons killed most of the Japanese soldiers as they charged across the sandbar, though a few of the Japanese soldiers reached across, cut through the barbed wire they found on the other end and engaged in hand-to-hand combat with the defenders, and captured a few of the Marine front-line emplacements. Japanese machine gun and rifle fire from the east side of the creek killed several of the Marine machine-gunners. A company of Marines, held in reserve just behind the front line, attacked and killed most, if not all, of the remaining Japanese soldiers that had breached the front line defenses, ending Ichiki's first assault about an hour after it had begun.
At 02:30 Ichiki ordered a second assault by the 1st and 3rd companies, and 150-200 Japanese troops again made a headlong attack, charging across the sandbar, and was again almost completely wiped out. A group of bold Japanese soldiers managed to swim in the creek and set up a machine gun on an abandoned amphibious tractor, their gunfire killed most of the Marines manning the 37mm gun that had wrought havoc on the attackers, and their continuous fire suppressed the gun for most of the battle. At least one of the surviving Imperial officers from this attack advised Ichiki to withdraw his remaining forces, but Ichiki declined to do so. This was the last concentrated charge across the sandbar, which had then become strewn with bodies of dead soldiers attempting to break through the marine defensive perimeter.