Islands, harbours, and navigable rivers were often held by the Norsemen, even when the near-by country was filled with Celts. The extreme north of Scotland, like the whole of the south, became Norse, as did the northern islands of Orkney and Shetland. Scapa Flow, that magnificent harbour in the Orkneys, was a stronghold of Norsemen many centuries before their descendants manned the British Grand Fleet there during the recent war. The Isle of Man was taken by Norsemen. Dublin, Waterford, and other Irish cities were founded by them. They attacked Wales from Anglessey; and, wherever they conquered, their armies were based on the sea. If you want to understand how the British Isles changed from a Celtic to a Nordic land, how they became the centre of the British Empire, and why they were the Mother Country from which the United States were born, you must always view the question from the sea. Take the sea as a whole, together with all that belongs to it--its islands, harbours, shores, and navigable rivers. Then take the roving Norsemen as the greatest seamen of the great seafaring Nordic race. Never mind the confusing lists of tribes and kings on either side--the Jutes and Anglo-Saxons, the Danes and Normans, on one side, and the Celts of England, Scotland, Wales, and Ireland, on the other; nor yet the different dates and places; but simply take a single bird's-eye view of all the Seven Seas as one sea, of all the British Norsemen as one Anglo-Norman folk, and of all the centuries from the fifth to the twentieth as a single age; and then you can quite easily understand how the empire of the sea has been won and held by the same strong "Hardy-Norseman" hands these fifteen hundred years.