This geographical factor restricted the theatre of major operations to some 180,000 square miles of sea. Minor offensive measures might have to be adopted against individual U-boats cruising at long distances from their bases, as actually occurred off the United States coast, but the fact of Germany possessing large submarine bases only along her own North Sea coast, and temporary ones on the Flanders littoral, enabled a concentration of Allied anti-submarine craft to be made in the narrow seas which afforded the only means of entry and exit to and from those bases. The same may be said of Austria in the Adriatic and of Turkey behind the Dardanelles. This favourable combination of circumstances would not occur if (however unthinkable) England, France or the United States were ever to wage a rigorous war against shipping. The large number of oversea naval bases possessed by these Powers would cause every sea to become a danger zone within a few hours of the commencement of hostilities. No effective concentration of hostile surface craft would be possible with the zone of operations spread over the water surface of the entire globe, and if the bases themselves were secured by predominant battle fleets, or numbers of heavily armed monitors, the seas would quickly become impossible for purposes of hostile transport. This geographical restriction of the German and Austrian danger zones made effective concentration of the Allied anti-submarine fleets and their devices possible. The 180,000 square miles of sea, forming the theatre of major operations, was, on special charts, divided into areas, comprising a few hundred square miles of sea. Each area was given a distinctive number, and a base was established for its own patrol and minesweeping fleet. The areas themselves were again subdivided on special charts into squares or sections. Each square covered a few leagues of sea and was known by an alphabetical sign. In this way the waters of the submarine danger zone were divided into areas, with their bases and protective fleets, and squares with their respective squadrons or ships. [Illustration: FIG. 20.--Diagram showing division of sea into anti-submarine patrol areas.] Each square of sea was covered once or twice daily by its own patrol ship or flotilla. Where the danger was less the patrol was not so frequent and the squares were almost indefinite in size, but where the chances of successful operations were exceptional, as in the Straits of Dover, additional offensive measures were employed (see under _Mine Barrages_).