The Kriegsmarine's most significant ships were the U-boats, most of which were constructed after Plan Z was abandoned at the beginning of World War II. Wolfpacks were rapidly assembled groups of submarines which attacked British convoys during the first half of the Battle of the Atlantic but this tactic was largely abandoned by May 1943 when U-boat losses mounted. Along with the U-boats, surface commerce raiders (including auxiliary cruisers) were used to disrupt Allied shipping in the early years of the war, the most famous of these being the heavy cruisers Admiral Graf Spee and Admiral Scheer and the battleship Bismarck.