On 9 December, two days after the Japanese attack on Pearl Harbor, "Mississippi" departed Iceland, bound for the Pacific Theater. She reached San Francisco on 22 January 1942, where she conducted training and escorted convoys along the west coast over the following seven months. Beginning in May 1942, the original 5-inch/51 caliber guns of the secondary battery were removed to make room for anti-aircraft machine guns. On 6 December, she escorted a convoy of troop ships to Fiji, returning to Pearl Harbor on 2 March 1943. Her first major combat operation began on 10 May, when she left Hawaii to support the liberation of the Aleutian Islands. She bombarded Kiska on 22 July, and the Japanese garrison withdrew from the island a few days later. After the conclusion of the campaign, "Mississippi" returned to San Francisco for an overhaul. On 19 October, she left San Pedro to join the invasion fleet that would attack the Gilbert Islands. During the Gilbert and Marshall Islands campaign on 20 November, while "Mississippi" was bombarding Makin, there was again an explosion in her No. 2 turret, this time killing 43 men. After repairs, she continued on in the campaign, bombarding Kwajalein on 31 January 1944, Taroa on 20 February, and Wotje on 21 February. On 15 March, she shelled Japanese positions at Kavieng on New Ireland, before returning to the United States for an overhaul in Puget Sound. This overhaul increased the number of 5-inch/25 cal guns from eight to fourteen.