Still in Shanghai at the beginning of the Pacific War on 8 December 1941, "Izumo" captured the American river gunboat and assisted in sinking the British river gunboat . On 31 December, the cruiser struck a mine in the Lingayen Gulf while supporting Japanese forces during the Philippines Campaign. During this period, "Iwate" was still serving as a training ship in home waters. The sisters were briefly re-classified as 1st-class cruisers on 1 July 1942 before they became training ships in 1943. "Izumo" returned to Japan late that year and joined her sister in training naval cadets.