WW1 saw the beginning of the end of Britain's dominance at sea. By far the most significant development was the submarine. The German U-Boats were able to challenge Britain's blockade of European waters, as well as take the conflict, usually undetected until it was too late, right to the American shoreline. Another weapon to make its debut in the latter years of the war was the aircraft carrier. Although converted from battleships and freighters, the first carriers played a vital part in shaping the vessels (and wars) of the future.