y ▁the ▁Japanese ▁use ▁of ▁the ▁islands ▁as ▁bases ▁for ▁threaten ing ▁the ▁supply ▁routes ▁between ▁the ▁U . S . ▁and ▁Australia , ▁and ▁to ▁secure ▁the ▁islands ▁as ▁starting ▁points ▁for ▁a ▁campaign ▁to ▁isol ate ▁the ▁major ▁Japanese ▁base ▁at ▁R aba ul ▁while ▁also ▁supporting ▁the ▁Al lied ▁New ▁Guinea ▁campaign . ▁The ▁land ings ▁initi ated ▁the ▁six - month - long ▁Gu adal can al ▁Camp aign . ▁ ▁The ▁Japanese ▁were ▁taken ▁by ▁surprise , ▁and ▁by ▁night fall ▁on ▁ 8 ▁August ▁the ▁ 1 1 , 0 0 0 ▁Al lied ▁troops — under ▁the ▁command ▁of ▁Lieutenant ▁General ▁Alexander ▁V ande gr ift — sec ured ▁Tul agi ▁and ▁nearby ▁small ▁islands ▁as ▁well ▁as ▁an ▁air field ▁under ▁construction ▁at ▁L ung a ▁Point ▁on ▁Gu adal can al . ▁The ▁Al lies ▁later ▁renamed ▁the ▁air field ▁Hend erson ▁Field . ▁To ▁protect ▁the ▁air field , ▁the ▁U . S . ▁Mar ines ▁established ▁a ▁per imeter ▁defense ▁around ▁L ung a ▁Point . ▁Add itional ▁rein for c ements ▁over ▁the ▁next ▁two ▁months ▁later ▁increased ▁the ▁number ▁of ▁U . S . ▁troops ▁at ▁L ung a ▁Point ▁to ▁more ▁than ▁ 2 0 , 0 0 0 . ▁ ▁In ▁response ▁to ▁the ▁Al lied ▁land ings ▁on ▁Gu adal can al , ▁the ▁Japanese ▁Imperial ▁General ▁Head quarters ▁assigned ▁the ▁Imperial ▁Japanese ▁Army ' s ▁ 1 7 th ▁Army — a ▁corps - s ized