With the outbreak of World War II in Europe on 1 September 1939, "Dallas" was recommissioned on 25 September 1939 and assigned to the United States Atlantic Fleet, serving as flagship for Destroyer Squadrons 41 and 30. She patrolled along the U.S. East Coast and conducted training exercises until 7 July 1941, when she got underway for Naval Station Argentia in the Dominion of Newfoundland, where she arrived on 11 July 1941. Between 11 July 1941 and 10 March 1942 she patrolled between Argentia and Halifax, Nova Scotia, and escorted convoys to Reykjavík, Iceland, and Derry, Northern Ireland.