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A strategically important natural harbor in the Orkney Islands, Scapa Flow served as Britain's main fleet anchorage during World Wars I and II. In 1914 and again in 1939, the British began building a comprehensive defensive network by fortifying the entrances to Scapa Flow, and then extended these defenses to cover most of Orkney. By 1940, it had become an island fortress, the largest integrated defensive network of its kind in Europe, manned by as many as 50,000 Commonwealth troops.