Germany at Sea!
The Kriegsmarine was the name of the German Navy between 1935 and 1945, during the Third Reich. The Kriegsmarine's most famous ships were the U-boat wolf packs. They were submarine groups which seriously harassed Allied convoys during the Battle of the Atlantic. U-boats and pocket battleships were used to disrupt Allied shipping in the earlier years of the war.
The U-boats were often seen as the biggest threat to Great Britain in the whole war. The U-boats continued operating successfully until mid 1943 when the Allies' air superiority, convoy system, and technological advances made U-boat operations unprofitable.