On 27 September 1913, Dönitz was commissioned as a "Leutnant zur See" (acting sub-lieutenant). When World War I began, he served on the light cruiser in the Mediterranean Sea. In August 1914, the "Breslau" and the battlecruiser were sold to the Ottoman Navy; the ships were renamed the "Midilli" and the "Yavuz Sultan Selim", respectively. They began operating out of Constantinople, under Rear Admiral Wilhelm Souchon, engaging Russian forces in the Black Sea. On 22 March 1916, Dönitz was promoted to "Oberleutnant zur See". He requested a transfer to the submarine forces, which became effective on 1 October 1916. He attended the submariner's school at Flensburg-Mürwik and passed out on 3 January 1917. He served as watch officer on , and from February 1918 onward as commander of . On 2 July 1918, he became commander of , operating in the Mediterranean. On 4 October, after suffering technical difficulties, Dönitz was forced to surface and scuttled his boat. He was captured by the British and incarcerated in the Redmires camp near Sheffield. He remained a prisoner of war until 1919 and in 1920 he returned to Germany.