As war swept across Europe, "Greer" recommissioned 4 October 1939, Commander J. J. Mahoney in command, and joined Destroyer Division 61 as flagship. After patrolling the East Coast and Caribbean, "Greer" joined the Neutrality Patrol in February 1940. Detached from this duty 5 October, the destroyer patrolled the Caribbean that winter. She joined other American ships on operations in the North Atlantic early in 1941, out of Reykjavík, Iceland, and NS Argentia, Newfoundland. United States ships, as non-belligerents, could not attack Axis submarines; but, as the German High Command stepped up the pace of the war through the summer of 1941, "Greer" found herself involved in an incident which brought America's entry into the war nearer.