The Kriegsmarine's most famous ships were the U-boats. Most of them were built after Plan Z was abandoned at the beginning of World War II. Wolfpacks were groups of submarines that attacked British convoys. Most of the attacks were during the first half of the Battle of the Atlantic. Along with the U-boats, commerce raiders and cruisers were used to disrupt Allied shipping in the early years of the war. The most famous of these were the heavy cruisers "Admiral Graf Spee" and "Admiral Scheer" and the battleship "Bismarck". However, the allied adoption of convoy escorts greatly reduced the danger from commerce raiders.