"Idaho" spent most of the 1920s and 1930s in the Pacific Fleet, where she conducted routine training exercises. Like her sister ships, she was modernized in the early 1930s. In mid-1941, before the United States entered World War II, "Idaho" and her sisters were sent to join the Neutrality Patrols that protected American shipping during the Battle of the Atlantic. After Japan attacked Pearl Harbor on 7 December 1941, "Idaho" and her sisters were sent to the Pacific, where she supported amphibious operations in the Pacific. She shelled Japanese forces during the Gilbert and Marshall Islands and the Philippines campaigns and the invasions of Peleliu, Iwo Jima, and Okinawa.