 Thus , Carrier Division 5 , consisting of the two most advanced aircraft carriers of the Kido Butai would not be available , which meant that Admiral Nagumo had to rely on four fleet carriers : Kaga and Akagi forming Carrier Division 1 and Hiryū and Sōryū as Carrier Division 2 . At least part of this was due to fatigue ; Japanese carriers had been constantly on operations since 7 December 1941 , including raids on Darwin and Colombo . 
