He began his career in the Imperial German Navy before World War I. In 1918, he was commanding UB-68 when she was sunk by British forces. Dönitz was taken prisoner. While in a prisoner of war camp, he formulated what he later called Rudeltaktik ("pack tactic", commonly called "wolfpack"). At the start of World War II, he was the senior submarine officer in the Kriegsmarine. In January 1943, Dönitz achieved the rank of Großadmiral (grand admiral) and replaced Grand Admiral Erich Raeder as Commander-in-Chief of the Navy.
On 30 April 1945, after the death of Adolf Hitler and in accordance with Hitler's last will and testament, Dönitz was named Hitler's successor as head of state, with the title of President of Germany and Supreme Commander of the Armed Forces. On 7 May 1945, he ordered Alfred Jodl, Chief of Operations Staff of the OKW, to sign the German instruments of surrender in Reims, France. Dönitz remained as head of the Flensburg Government, as it became known, until it was dissolved by the Allied powers on 23 May.
Despite his postwar claims, Dönitz was seen as supportive of Nazism during the war, and he is known to have made a number of anti-Semitic statements. Following the war, Dönitz was indicted as a major war criminal at the Nuremberg Trials on three counts: (1) conspiracy to commit crimes against peace, war crimes, and crimes against humanity; (2) planning, initiating, and waging wars of aggression; and (3) crimes against the laws of war. He was found not guilty on count (1) of the indictment, but guilty on counts (2) and (3). He was sentenced to ten years' imprisonment; after his release, he lived in a village near Hamburg until his death in 1980. For nearly seven decades, Dönitz was the only head of state to be convicted by an international tribunal until the conviction of Liberia's Charles Taylor in April 2012.