At the time of the attack on Pearl Harbor, "Isonami" was assigned to Destroyer Division 19 of Desron 3 of the IJN 1st Fleet, and had deployed from Kure Naval District to the port of Samah on Hainan Island. From 4 December 1941 to 30 January 1942 "Isonami" was part of the escort for the heavy cruisers , , , and out of Samah and Camranh Bay, French Indochina in support of Malaya, Banka-Palembang and Anambas Islands invasion operations. On 27 February, "Isonami" was assigned to "Operation J" (the invasion of Java), and "Operation T" (the invasion of northern Sumatra) on 12 March and the "Operation D" (the invasion of the Andaman Islands on 23 March. She served patrol and escort duties out of Port Blair during the Japanese raids into the Indian Ocean. On 13–22 April "Isonami" returned via Singapore and Camranh Bay to Kure Naval Arsenal, for maintenance.