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.Hogue, Pearl Harbor to Guadalcanal, pp. 235–236.
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.Morison, Struggle for Guadalcanal, pp. 14–15. 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.Zimmerman, The Guadalcanal Campaign, pp. 49–56.Smith, Bloody Ridge, pp. 11, 16.
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.Shaw, First Offensive, p. 13.Smith, Bloody Ridge, pp. 16–17.
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.Miller, The First Offensive, p. 96 The different units began to move towards Guadalcanal immediately, but Ichiki's regiment, being the closest, arrived first.Smith, Bloody Ridge, p. 88; Evans, Japanese Navy, p. 158; Frank, Guadalcanal, pp. 141–143. The Ichiki regiment was named after its commanding officer and was part of the 7th Division from Hokkaido. The Aoba regiment, from the 2nd Division, took its name from Aoba Castle in Sendai, because most of the soldiers in the regiment were from Miyagi prefecture (Rottman, Japanese Army, p. 52). Ichiki's regiment had been assigned to invade and occupy Midway Atoll, but were on their way back to Japan after the invasion was canceled following the Japanese defeat in the Battle of Midway. Although some histories state that Ichiki's regiment was at Truk, Raizo Tanaka, in Evans' book, states that he dropped off Ichiki's regiment at Guam after the Battle of Midway. Ichiki's regiment was subsequently loaded on ships for transport elsewhere but were rerouted to Truk after the Allied landings on Guadalcanal.
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.Frank, Guadalcanal, pp. 143–144. 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."Evans, Japanese Navy, p. 161; Griffith, Battle for Guadalcanal, pp. 98–99; Smith, Bloody Ridge, p. 31.
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 east of Lunga Point, by six destroyers at 01:00 on 19 August.Evans, Japanese Navy, p. 161; Frank, Guadalcanal, p. 145; Jersey, Hell's Islands, pp. 204, 212; Morison, Struggle for Guadalcanal, p. 70; Smith, Bloody Ridge, p. 43. The First Element troops were mainly from the 28th's 1st Battalion under a Major Kuramoto and were mostly from Asahikawa, Hokkaidō. At Taivu Point was an Imperial outpost with about 200 naval personnel who assisted with the unloading of Ichiki's forces from the destroyers. Ichiki was ordered to scout the American positions and wait for the remainder of his force to arrive. Known as the (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."Spector, Eagle Against the Sun, p. 496 He concocted a brazenly simple plan: march straight down the beach and through the American defenses.Gilbert, Marine Tank Battles in The Pacific, p. 41 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 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.Griffith, Battle for Guadalcanal, pp. 99–100; Smith, Bloody Ridge, pp. 29, 43–44.
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.Frank, Guadalcanal, p. 148; Jersey, Hell's Islands, p. 205.Zimmerman, The Guadalcanal Campaign, p. 62.
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.Griffith, Battle for Guadalcanal, p. 100; Jersey, Hell's Islands, p. 205; Smith, Bloody Ridge, p. 47. The U.S. and Japanese soldiers killed in this engagement are included in the total casualty figures for the Tenaru battle. Captain Yoshimi Shibuya was the leader of the Japanese patrol. One of the five Japanese survivors later died of his wounds at Taivu Point. 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.Zimmerman, The Guadalcanal Campaign, p. 62 The papers did not detail exactly how large the Japanese force was or whether an attack was imminent.Frank, Guadalcanal, p. 149.