 Betio was the site of a crucial Japanese airfield.To protect it from capture, Keiji had 14 coastal defense guns, 50 pieces of field artillery, 100 machine gun nests, and 500 pillboxes installed, as well as a large wall built across the northern lagoon.After the Gilberts fell to the Americans in late November 1943, Admiral Mineichi Koga of the Japanese Combined Fleet was unsure of which islands the Americans would strike.