 She became flagship of the Second Division from 22 September to 26 October while "Sōryū" was refitting.In November 1941, the IJN's Combined Fleet, commanded by Admiral Isoroku Yamamoto, prepared to participate in Japan's initiation of a formal war with the United States by conducting a preemptive strike against the United States Navy's Pacific Fleet base at Pearl Harbor, Hawaii.On 22 November, "Hiryū", commanded by Captain Tomeo Kaku, and the rest of the "Kido Butai", under Vice Admiral Chuichi Nagumo and including six fleet carriers from the First, Second, and Fifth Carrier Divisions, assembled in Hitokappu Bay at Etorofu Island.