The rapid development of these raiding enterprises has been explained as caused by political changes in Norway. But these changes did not take place until about eighty years after the Norse raids began. They amounted to a strong centralisation under king Harald the Fairhaired and a diminution of the power of the nobles; and they were perhaps rather a consequence than a cause of the raiding movement. We have seen how, some five centuries earlier, an almost similar outbreak of raiding activity brought the Irish into touch with Roman Britain and Gaul, and how the rewards of plunder enabled Irish kings to maintain a permanent military organisation and to acquire thereby much greater power, leading to a depression of the old nobility. I think it likely that the chief cause of the Norse movement of invasion was the development of a particularly suitable style of ship-building; the building of long undecked ships of light draft and very strong construction, very seaworthy; in which, during a sea-fight, every man could take a hand. The third phase was the occupation of inland waters. The invaders ran their ships, which were propelled by oars as well as by sails, up the navigable rivers, if necessary dragging them overland where the navigable parts were interrupted by shallow rapids, for example on the Bann and the Shannon. Thus they could place a whole fleet on a lake like Loch Neagh or Loch Ree. There they were safe from attack and were in a position to choose the place on a large shoreline for their incursions. It is to be borne in mind that, during the period of the Norse wars in Ireland and for some centuries before and after it, the Irish had no permanent military organisation. Their largest military operations never extended beyond a few weeks. Their fighting men were called out for the purpose from their ordinary peaceful occupations, and could not lawfully be held to military service for more than a few weeks in any year. Thus there was no effective means of fighting down a hostile force encamped on its ships in a large inland water. It was by a crafty lure, we are told, that Turgesius, commander of the Norse fleet on the Shannon, was captured.