Underestimating the strength of Allied forces on Guadalcanal , Ichiki 's First Element conducted a nighttime frontal assault on Marine positions at Alligator Creek on the east side of the Lunga perimeter in the early morning hours of August 21 . Ichiki 's assault was repulsed with devastating losses for the attackers in what became known as the Battle of the Tenaru : all but 128 of the 917 men of the First Element ( including Ichiki himself ) were killed in the battle . The survivors returned to Taivu Point , notified 17th Army headquarters of their defeat in the battle and awaited further reinforcements and orders from Rabaul .