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.

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.

[7] 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.

Major General Alexander Vandegrift placed his 11,000 troops on Guadalcanal in a loose perimeter around the Lunga Point area.

[15] 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.

[19] 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 14 kilometers (8.7 mi) east of the Lunga perimeter.

[24] 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.

Several official U.S. military histories identify the location of the eastern defenses of the Lunga perimeter as emplaced on the Tenaru River.

[28][29] To help further defend the Alligator Creek sandbar, Cates deployed 100 men from the 1st Special Weapons Battalion with two 37 mm anti-tank guns equipped with canister shot.

[32] Just after midnight on 21 August, Ichiki's main body of troops arrived at the east bank of Alligator Creek and were surprised to encounter the Marine positions, not having expected to find U.S. forces so far from the airfield.

[33] Nearby U.S. Marine listening posts heard "clanking" sounds, human voices, and other noises before withdrawing to the west bank of the creek.

[34] Marine machine gun fire and canister rounds from the 37 mm cannons killed most of the Japanese soldiers as they crossed the sandbar.

[41][42] For the next couple of hours, the two sides exchanged rifle, machine gun, and artillery fire at close range across the sandbar and creek.

[43] In spite of the heavy losses his force had suffered, Ichiki's troops remained in place on the east bank of the creek, either unable or unwilling to withdraw.

[44] At daybreak on 21 August, the commanders of the U.S. Marine units facing Ichiki's troops conferred on how best to proceed, and they decided to counterattack.

[45] The 1st Battalion, 1st Marine Regiment, under Cresswell, crossed Alligator Creek upstream from the battle area, enveloped Ichiki's troops from the south and east, cutting off any avenue for retreat, and began to "compress" Ichiki's troops into a small area in a coconut grove on the east bank of the creek.

[43] Aircraft from Henderson Field strafed Japanese soldiers who attempted to escape down the beach and, later in the afternoon, four or five Marine M3 Stuart tanks attacked across the sandbar into the coconut grove.

The tanks swept the coconut grove with machine gun and canister cannon 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.

Colonel Ichiki was either killed during the final stages of the battle, or performed ritual suicide (seppuku) shortly thereafter, depending on the account.

"[53] Robert Leckie, a Guadalcanal veteran, recalls the aftermath of the battle in his book Helmet for My Pillow, "Our regiment had killed something like nine hundred of them.

Reacting with disbelief to the news, Japanese Army headquarters officers proceeded with plans to deliver additional troops to Guadalcanal to reattempt to capture Henderson Field.