What is "sea frontier"

Sea Frontiers were established by the United States Navy from 1 July 1941 during World War II as areas of defense against enemy vessels, especially submarines, along the American coasts. Sea Frontiers generally started at the shore of the United States and extended outwards into the sea for a nominal distance of two hundred miles.

As early as 1927 the Navy's plans for the coastal defense of the United States and its Territories and possessions provided for the establishment of Naval Coastal Frontiers that would be larger operational commands than the individual Naval Districts. On July 1, 1941, the Chief of Naval Operations formally established several Naval Coastal Frontiers; on February 6, 1942, these were renamed Sea Frontiers. Each Frontier was a geographic area, usually comprising a number of Naval Districts but including in addition the outer shipping lanes in its sea area. The land areas of the Frontiers corresponded roughly to the Army's Defense Commands, but the boundaries were not identical. The Frontier Commander was usually also the commandant of a Naval District within the Frontier. The chief responsibilities of the Sea Frontiers during World War II were operational; Frontier forces engaged actively in scouting for enemy forces, particularly submarines, and in attack on any enemy units within their boundaries. Toward the end of the war the Frontiers were assigned administrative and logistic functions in addition to their operational responsibilities.

Navy General Order No. 143, issued on February 3, 1941, stated that Commandants of United States naval districts and Commanders of Naval Coastal Frontiers have administrative responsibility direct to the Navy Department for local and coastal forces; but Commanders of Naval Coastal Frontiers have task responsibility to the Chief of Naval Operations for Naval Coastal Frontier Forces. (Source Eastern Sea Frontier history, HyperWar)

In addition to the Sea Frontiers under the cognizance of U.S. military authorities, the Canadian Coastal Zone was the responsibility of the Royal Canadian Navy. This formation was very active since the majority of trans-Atlantic convoys originated or terminated in Canadian waters.