The ship remained in North American waters during World War I, conducting training exercises to work up the crew. Throughout the 1920s and 1930s, the ship served in the Pacific Fleet. In May 1941, with World War II and the Battle of the Atlantic raging, "Mississippi" and her two sister ships were transferred to the Atlantic Fleet to help protect American shipping through the Neutrality Patrols. Two days after the Japanese attack on Pearl Harbor, "Mississippi" departed the Atlantic to return to the Pacific Fleet; throughout her participation in World War II, 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 and Okinawa. The Japanese fleet attacked American forces during the Philippines campaign, and in the ensuing Battle of Leyte Gulf, "Mississippi" took part in the Battle of Surigao Strait, the last battleship engagement in history.