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 August 21, 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, which guarded Henderson Field, which had been captured by the Allies in landings on Guadalcanal on August 7. Ichiki's unit was sent to Guadalcanal in response to the Allied landings 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 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. All but 128 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 sent larger forces to the island for their subsequent attempts to retake Henderson Field.



The Guadalcanal campaign, also known as the Battle of Guadalcanal and codenamed Operation Watchtower by American forces, was a military campaign fought between 7 August 1942 and 9 February 1943 on and around the island of Guadalcanal in the Pacific theater of World War II. It was the first major land offensive by Allied forces against the Empire of Japan.

On 7 August 1942, Allied forces, predominantly United States Marines, landed on Guadalcanal, Tulagi, and Florida in the southern Solomon Islands, with the objective of using Guadalcanal and Tulagi as bases in supporting a campaign to eventually capture or neutralize the major Japanese base at Rabaul on New Britain. The Japanese defenders, who had occupied those islands since May 1942, were outnumbered and overwhelmed by the Allies, who captured Tulagi and Florida, as well as the airfield – later named Henderson Field – that was under construction on Guadalcanal.

Surprised by the Allied offensive, the Japanese made several attempts between August and November to retake Henderson Field. Three major land battles, seven large naval battles (five nighttime surface actions and two carrier battles), and almost daily aerial battles culminated in the decisive Naval Battle of Guadalcanal in early November, with the defeat of the last Japanese attempt to bombard Henderson Field from the sea and to land enough troops to retake it. In December, the Japanese abandoned their efforts to retake Guadalcanal, and evacuated their remaining forces by 7 February 1943, in the face of an offensive by the U.S. Army's XIV Corps, with the Battle of Rennell Island, the last major naval engagement, serving to secure protection for the Japanese troops to evacuate safely.

The campaign followed the successful Allied defensive actions at the Battle of the Coral Sea and the Battle of Midway in May and June, 1942. Along with the battles at Milne Bay and Buna–Gona, the Guadalcanal campaign marked the Allies' transition from defensive operations to offensive ones and effectively allowed them to seize the strategic initiative in the Pacific theater from the Japanese. The campaign was followed by other Allied offensives in the Pacific, most notably: the Solomon Islands campaign, New Guinea campaign, the Gilbert and Marshall Islands campaign, the Mariana and Palau Islands campaign, the Philippines campaign (1944–1945), and the Volcano and Ryukyu Islands campaign prior to the surrender of Japan in August, 1945.