At the outbreak of the First World War in 1914, she was flagship of the 4th Battle Squadron in the North Sea, based at Scapa Flow. She was relieved as flagship on 10 December by . Ironically for a vessel designed to engage enemy battleships, her only significant action was the ramming and sinking of German submarine , skippered by K/Lt Otto Weddigen (of fame), on 18 March 1915. "U-29" had broken the surface immediately ahead of "Dreadnought" after firing a torpedo at "Neptune", and "Dreadnought" cut the submarine in two after a short chase. She almost collided with who was also attempting to ram the submarine. "Dreadnought" thus became the only battleship ever to purposefully sink an enemy submarine.