 The formed the primary carrier fleet of the Imperial Japanese Navy (IJN), a grouping of naval aircraft and aircraft carriers that at the time of the attack on Pearl Harbor, was the world's largest aircraft carrier fleet.As losses mounted, the carriers were removed and the 1st Air Fleet was transformed into a land based Naval aviation force forward based on islands and land masses along the perimeter of the Japanese controlled sphere.The was organized late in the War to defend the National Defence Zone along with 1st Air Fleet.