Battle of the Tenaru

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, which 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 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.