Passage: Derfflinger was part of the I Scouting Group for most of World War I, and was involved in several fleet actions during the war. She took part in the bombardments of English coastal towns, as well as the Battles of Dogger Bank and Jutland, where her stubborn resistance led to the British nicknaming her "Iron Dog". The ship was partially responsible for the sinking of two British battlecruisers at Jutland; Derfflinger and Seydlitz destroyed Queen Mary, and Lützow assisted her elder sister in the sinking of Invincible. Derfflinger was interned with the rest of the High Seas fleet at Scapa Flow following the armistice in November 1918. Under the orders of Rear Admiral Ludwig von Reuter, the interned ships were scuttled on 21 June 1919; Derfflinger sank at 14:45.

Question: What was Derfflinger's nickname?
Iron Dog