"New Mexico" and "Idaho" took part in the Mariana and Palau Islands campaign beginning in June 1944, though "Mississippi" was out of service for a refit during the first part of the campaign. They shelled Japanese positions on Saipan, Tinian, and Guam as amphibious forces assaulted each island in turn. The two battleships remained with the invasion fleet to cover it from Japanese attacks while the Fast Carrier Task Force met and decisively defeated the Japanese 1st Air Fleet in the Battle of the Philippine Sea. By September, "Mississippi" had returned to the fleet to relieve "New Mexico" for her own refit, and she and "Idaho" shelled the island of Pelelieu, the last battle of the Marianas campaign. The fleet thereafter withdrew to Manus Island to prepare for the coming invasion of the Philippines, while "New Mexico" departed for a refit at the Puget Sound Navy Yard.