"Hōshō" began the Pacific War in the Third Carrier Division assigned to the 1st Fleet under Vice Admiral Shirō Takasu. The carrier, captained by Kaoru Umetani, was tasked along with to provide air support, including scouting, anti-submarine patrols, and combat air patrol for the Combined Fleet's "Main Body" battle-line of six battleships: , , , , , and . With the Main Body, "Hōshō" sortied from the Inland Sea on 7 December 1941 to provide distant cover for the carrier forces under Chūichi Nagumo which were attacking Pearl Harbor. The battleship force turned back east of Japan, but "Hōshō" became separated on 10 December due to radio silence restrictions while conducting anti-submarine air operations. The ship lost contact because she had launched aircraft near dusk to investigate a submarine sighting. The aircraft returned after dark and landed safely after the carrier turned on its lights. In order to recover the aircraft, however, the carrier and her three escorting destroyers had to steam east and therefore lost sight of the Main Body in the darkness. There does not appear to have been any US or other enemy submarines in Japanese waters at this time. The carrier was located by scout aircraft the next day east of the Main Body and returned to port at Kure on 12 December.