Pan-American Security Zone
During the early years of World War II before the United States became a formal belligerent, President Franklin D. Roosevelt declared a region of the Atlantic, adjacent to the Americas as the Pan-American Security Zone. Within this zone, United States naval ships escorted convoys bound for Europe. In practice, this greatly aided the United Kingdom, which was largely dependent upon the Atlantic convoys.
The Zone was one of a number of actions taken by the United States that ran counter to its formal state of neutrality. It was set up in October 1939 at US behest by the Declaration of Panama signed by the nations of North and South America. Within the Zone which extended between 300 and 1,000 nautical miles (560 and 1,850 km; 350 and 1,150 mi) offshore, the signatories would not tolerate belligerent acts.
Despite formal complaints in December 1939 to Britain over the action against the Admiral Graf Spee off the River Plate, US implementation of the Zone was clearly to Britain's advantage. From early 1941, United States Navy convoy escorts eased British and Canadian difficulties by providing escorts; USN ships and aircraft were ordered to broadcast in clear any U-boat sightings, thus alerting British listeners. The Kriegsmarine (German navy) resented this "cheating" but they were instructed to avoid hostile acts against US ships so as not to give cause for a declaration of war.
On 18 April 1941, Roosevelt extended the Pan-American Security Zone to longitude 26 degrees west, 2,300 nautical miles (4,300 km; 2,600 mi) east of New York and just 50 nautical miles (93 km; 58 mi) short of Iceland, a major convoy staging area.
Original text of the Panama Declaration and associated documents (including map) can be found at the following link: http://digital.library.wisc.edu/1711.dl/FRUS.FRUS1939v05. Original source: United States Department of State, Foreign Relations of the United States diplomatic papers, 1939, volume V, The American Republics, (Washington, DC: Government Printing Office, 1939), pp. 34-39