The 'Battle of the Tenaru', sometimes called the 'Battle of the Ilu River' or the 'Battle of Alligator Creek', was fought between Japanese and US forces on Guadalcanal island of the Solomon islands group during the Pacific War (21 August 1942)

The battle was the first major Japanese land offensive during the Guadalcanal campaign resulting from the US 'Watchtower' landings on 7 August 1942.

In the 'Battle of the Tenaru', marines of Major General Alexander A. Vandegrift’s 1^st Marine Division repulsed an assault by the First Element of the 'Ichiki' Regiment commanded by Colonel Kiyonao Ichiki. The marines were defending the Lunga perimeter round Henderson Field, which had been captured by the US forces in the landings of 7 August. Ichiki’s unit had been despatched to Guadalcanal in direct response to the US landings, with the mission of recapturing the airfield and driving the Allied forces off the island.

Underestimating the strength of US forces on Guadalcanal, which at the time numbered about 11,000 men, Ichiki’s unit conducted a nocturnal frontal assault on the marine positions at Alligator Creek on the eastern side of the Lunga perimeter. Jacob Vouza, a local coastwatcher scout, warned the Americans of the impending attack minutes before Ichiki’s assault, and the Japanese were defeated with heavy losses. The marines counterattacked Ichiki’s surviving troops after the arrival of day, killing many more. About 800 of the original 917 men of the 'Ichiki' Regiment died.

The battle was the first of three separate major land offensives by the Japanese in the Guadalcanal campaign. The Japanese realised after the 'Battle of the Tenaru' that the US 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.

It was on 7 August 1942 that US forces had landed on Guadalcanal and the nearby Tulagi and Florida islands toward the south-eastern end of the Solomon islands group. The landings were designed to deny their use by the Japanese as bases for threatening the maritime supply routes between the USA and Australia, and to secure the islands as launch points for a campaign with the eventual goal of isolating the major Japanese base at Rabaul on the island of New Britain while also supporting the Allied land campaign on New Guinea. The landings initiated the six-month Guadalcanal campaign.

Taking the Japanese by complete operational and tactical surprise, the US landing forces accomplished their initial objectives of securing Tulagi and nearby small islands, as well as the airfield currently under construction by the Japanese at Lunga Point on Guadalcanal’s northern coast, by the fall of night on 8 August. That night, as the transport vessels unloaded, the Allied warships screening the transports were surprised and defeated by an Imperial Japanese naval force of seven cruisers and one destroyer under the command of Vice Admiral Gunichi Mikawa. One Australian and three US cruisers were sunk and one other US cruiser and two destroyers were damaged in the resulting 'Battle of Savo Island'. Rear Admiral Richmond K. Turner had withdrawn the remaining Allied naval forces by the evening of 9 August without unloading all the heavy equipment, provisions and troops from the transport vessels, although most of the divisional artillery, comprising 32 75-mm (2.95-in) and 105-mm (4.13-in) howitzers, had been brought ashore. Rations for only five days had been landed.

The marines on Guadalcanal initially concentrated on forming a defensive perimeter round the airfield, moving the landed supplies within the perimeter, and completing construction of the airfield. Vandegrift placed his 11,000 men 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 immediately on the completion of the airfield using captured Japanese equipment. On 12 August, the airfield was named Henderson Field after Major Lofton Henderson, a marine pilot who had been killed at the 'Battle of Midway'. Captured Japanese stocks increased the availability of food to 14 days although, to conserve the limited supply, the US troops were limited to two meals per day.

Responding to the US landings on Guadalcanal, the Imperial General Headquarters assigned to the 17^th Army, a corps-sized formation based at Rabaul under the command of Lieutenant General Harukichi Hyakutake, the task of retaking Guadalcanal. Currently involved heavily in the Japanese campaign in New Guinea, the 17^th Army had only a few units available for immediate despatch to the area at the south-eastern end of the Solomon islands group. Of these units, the 35^th Brigade, under the command of Major General Kiyotake Kawaguchi, was in the Palau islands group, the 4^th ('Aoba') Regiment was in the Philippine islands group, and the 28^th ('Ichiki') Regiment, under the command of Ichiki, was at sea en route to Japan from Guam. These physically disparate units immediately began to move toward Guadalcanal and, being closest, Ichiki’s regiment arrived first.

Flown on 12 August, an air reconnaissance of the marine positions on Guadalcanal with a senior Japanese staff officer from Rabaul on board, sighted few US troops in the open and no large ships in the nearby waters, and this convinced the Imperial General Headquarters that the Allies had withdrawn the majority of their troops: in reality, no Allied troops had been withdrawn. Hyakutake issued orders for an advance unit of 900 men of Ichiki’s regiment to be landed on Guadalcanal by fast warship for an immediate attack on the US position and the reoccupation of the airfield area at Lunga Point. The rest of Ichiki’s regiment was to be delivered to Guadalcanal later in slower transport ships. At the major Japanese naval base at Truk in the Caroline islands group, which was the staging point for the delivery of Ichiki’s regiment to Guadalcanal, Ichiki was briefed that between 2,000 and 10,000 US troops were holding the Guadalcanal beach-head and that he should 'avoid frontal attacks'.

Together with 916 of his regiment’s 2,300 men, Ichiki designated his force as the First Element, and this was delivered, with a seven-day supply of food, to Taivu Point, about 22 miles (35 km) to the east of Lunga Point, by six destroyers at 01.00 on 19 August. Ichiki was ordered to scout the US positions and await the arrival of the rest of his force. Known as the 'Ichiki' Detachment, this was an elite and battle-hardened force but, as was about to be discovered, was strongly stricken with 'victory disease', otherwise overconfidence resulting from previous success. Ichiki was so confident in the superiority of his men that he decided to destroy the US defenders before the 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 created an openly simple plan: march straight along the beach and strike through the US defences. Leaving about 100 men as a rearguard, Ichiki marched to the west with the remaining 800 men of his unit and made camp before dawn about 8.7 miles (14 km) to the east of the Lunga Point perimeter. The marines at Lunga Point received intelligence that a Japanese landing had occurred and took steps to find out exactly what was taking place.

Reports by 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, along with Allied intelligence from other sources, indicated that there were Japanese troops to the east of Lunga Point. for further investigation, on 19 August a 60-man marine patrol, together with four islander scouts, under the command of Captain Charles H. Brush, departed to the east from the Lunga perimeter.

At the same time, Ichiki sent forward his own patrol of 38 men, led by his communications officer, to reconnoitre the US troop dispositions and establish a forward communications base. At about 12.00 on 19 August, at Koli Point, Brush’s patrol sighted and ambushed the Japanese patrol, killing all but five of its men, who escaped back to Taivu. The marines lost three killed and three wounded.

Papers discovered on the bodies of some of the Japanese patrol’s officers revealed that the ambushed men were part of a considerably larger unit and showed detailed intelligence of marine positions around Lunga Point. However, the captured papers did not detail the exact strength of the Japanese force or whether or not an attack was imminent.

Now anticipating an attack from the east, the marines, under Vandegrift’s direction, prepared their defences on the eastern side of the Lunga Point perimeter. Several US official histories identify the location of the these eastern defences as the Tenaru river, but this river is in fact located farther to the east. The river forming the eastern boundary of the Lunga Point perimeter is the Ilu river, nicknamed Alligator Creek by the marines: this was a double misnomer as there are crocodiles not alligators in the Solomon islands group, and the so-called Alligator Creek is in fact a tidal lagoon separated from the sea by a sandbar about 23 to 49 ft (7 to 15 m) wide and 98 ft (30 m) long.

Along the western side of Alligator Creek, Colonel Clifton B. Cates, commander of the 1^st Marine Regiment, deployed his 1^st Battalion (Lieutenant Colonel Leonard B. Cresswell) and 2^nd Battalion (Lieutenant Colonel Edwin A. Pollock). The greater to support the defence along the Alligator Creek sandbar, Cates deployed 100 men of the 1^st Special Weapons Battalion with two 37-mm anti-tank guns equipped with canister anti-personnel ammunition. Marine divisional artillery, comprising both 37- and 105-mm (2.95- and 4.13-in) howitzers, pre-targeted locations on the eastern side and sandbar areas of Alligator Creek, and forward artillery observers emplaced themselves in the forward marine positions. The marines worked all through 20 August to prepare their defences as much as possible before the fall of night.

Learning of the annihilation of his patrol, Ichiki quickly sent forward one company to bury the dead 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 marine positions on the eastern side of Lunga Point. At this location, he prepared his troops to attack the US positions that night.

Just after 00.00 on 21 August, Ichiki’s main body reached he eastern bank of Alligator Creek and was surprised to encounter the marine positions as it had not expected to find US forces so far from the airfield. Before pulling back onto the western bank of the creek, nearby marine listening posts heard 'clanking' sounds, human voices and other noises. At 01.30 Ichiki’s force opened fire with machine guns and mortars on the marine’s western-bank positions, and a first wave of about 100 men charged across the sandbar toward the marines. Machine gun fire and canister rounds from the 37-mm guns killed most of the Japanese as they crossed the sandbar, but a few Japanese reached the marine positions, engaged in hand-to-hand combat with the defenders, and took a few of the marines' front-line emplacements. Japanese machine gun and rifle fire from the creek’s eastern side killed several 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 who had breached the front-line defences, ending Ichiki’s first assault about an hour after it had begun.

At 02.30 a second wave of about 150 to 200 Japanese again attacked across the sandbar and was again almost completely destroyed. At least one of the surviving Japanese officers who survived this attack advised Ichiki to withdraw his remaining men, but Ichiki decided not to do so.

As Ichiki’s men regrouped to the east of the creek, Japanese mortars bombarded the marine lines. The marines answered with 75-mm (2.95-in) artillery and mortar barrages into the areas to the east of the creek. At about 05.00, another wave of Japanese troops attacked, this time attempting to flank the marine positions by wading through the sea surf to attack up the beach into the western-bank area of the creek bed. The marines responded with heavy machine gun and artillery fire along the beach-front area, again inflicting heavy losses on Ichiki’s attacking troops and causing them to abandon their attack and withdraw to the eastern bank. For the next two hours, the sides exchanged rifle, machine gun and artillery fire at close range across the sandbar and creek.

Despite his men’s heavy losses, Ichiki’s troops remained on the east bank of the creek, either unable or unwilling to withdraw. As day broke on 21 August, the marine commanders facing Ichiki’s force conferred on how best to proceed, and decided to counterattack. The 1/1^st Marines, under Cresswell, crossed Alligator Creek upstream of the battle area, enveloped Ichiki’s force from the south and east, thereby cutting off any avenue of retreat, and began to 'compress' Ichiki’s troops into a small area in a coconut grove on the eastern bank of the creek.

Warplanes from Henderson Field strafed Japanese troops attempting to escape down the beach and, later in the afternoon, four or five of the marines' M3 Stuart light tanks attacked across the sandbar into the coconut grove. The tanks swept the grove with machine gun and canister gun fire, as well as rolling over the bodies, both alive and dead, of any Japanese soldiers unable or unwilling to get out of the way. When the tank attack was over, Vandegrift wrote that 'the rear of the tanks looked like meat grinders'.

By 17.00 on 21 August, Japanese resistance had come to an end. Ichiki was either killed during the final stages of the battle, or performed ritual suicide soon after it. As curious marines began to walk around looking at the battlefield, some wounded Japanese troops opened fire, killing or wounding several of them. Thereafter, marines shot and/or bayoneted any Japanese soldier lying on the ground who moved. About 15 injured and unconscious Japanese soldiers were taken prisoner. Some 30 of the Japanese escaped to rejoin their regiment’s rear echelon at Taivu Point, but in overall terms about 800 Japanese were killed in the fighting.

For the USA and its allies, the victory in the 'Battle of the Tenaru' was psychologically significant inasmuch as Allied soldiers, after a series of defeats by Japanese army units throughout the Pacific and east Asia, now knew that they could hold and defeat the Japanese in a land battle. The episode set another precedent that would continue throughout the Pacific War, which was the reluctance of defeated Japanese to surrender and their efforts to continue killing Allied soldiers, even as the Japanese lay dying on the battlefield. On this subject Vandegrift remarked that 'I have never heard or read of this kind of fighting. These people refuse to surrender. The wounded wait until men come up to examine them…and blow themselves and the other fellow to pieces with a hand grenade.'

By 25 August, most of the survivors of Ichiki’s force had reached Taivu Point and radioed Rabaul to inform the 17^th Army's headquarters that Ichiki’s detachment had been 'almost annihilated at a point short of the airfield'. Reacting with disbelief to the news, Japanese headquarters officers proceeded with plans to deliver additional troops to Guadalcanal to reattempt to capture Henderson Field. The next major Japanese attack on the Lunga Point perimeter occurred at the 'Battle of Edson’s Ridge' some three weeks later, employing a significantly larger force than had been employed at Tenaru, and coming much closer to victory.