On 7 December 1941 "Sumner" was moored at the Submarine Base, Pearl Harbor, and took an active part in the defense of the Islands against the Japanese attack. After the attack she assisted the stricken ships in the area. On 12 January 1942 she set sail for Tongatapu and thence to Nandi Island and Samoa for surveying. After transporting Marines to Wallis Island in May, she made a survey of local harbors. During the ensuing months she conducted surveys at Noumea, New Caledonia; Nukualofa, Tonga; Port Vila and Espiritu Santo, New Hebrides; and Tulagi, Solomon Islands.