Taking the Japanese by surprise , by nightfall on 8 August the Allied landing forces had secured Tulagi and nearby small islands , as well as an airfield under construction at Lunga Point ( 9 ° 25 ′ 45 ″ S 160 ° 3 ′ 4 ″ E ) on the north shore of the island of Guadalcanal east of the present day capital of Honiara . Vandegrift placed his 11 @,@ 000 troops of the 1st Marine Division on Guadalcanal in a loose perimeter around the Lunga Point area .