Between the land and sea battles, more sailors than marines were KIA in the Guadalcanal campaign, about 4,500 to 1,500.
History can thank the CNO Admiral Earnest King for taking the USN to war in the Pacific so soon. Under the “Europe First” strategy, Churchill and FDR were willing to give up Australia, NZ and India if necessary to beat Hitler first.
Admiral King didn’t get the memo.
There was never any doubt that the US would fight the Japanese (almost typed "Japs" there) -- George Marshall wanted an early second front in Europe, was resisted by the UK every step of the way, but the US threat (not usually veiled, either), was that the US needed all the resources it could raise to fight alone in the Pacific. After V-E day, there was resistance from the US command structure to allow the British Navy to join the action in Okinawa.