The defense by his enemies!
On the specific war crimes charge of ordering unrestricted submarine warfare, Dönitz was found "[not] guilty for his conduct of submarine warfare against British armed merchant ships", because they were often armed and equipped with radios which they used to notify the Admiralty of attack but the judges found, "Dönitz is charged with waging unrestricted submarine warfare contrary to the Naval Protocol of 1936 to which Germany acceded, and which reaffirmed the rules of submarine warfare laid down in the London Naval Agreement of 1930... The order of Dönitz to sink neutral ships without warning when found within these zones was, therefore, in the opinion of the Tribunal, violation of the Protocol... The orders, then, prove Dönitz is guilty of a violation of the Protocol... the sentence of Dönitz is not assessed on the ground of his breaches of the international law of submarine warfare."
His sentence on unrestricted submarine warfare was not assessed, because of similar actions by the Allies: in particular, the British Admiralty on 8 May 1940 had ordered all vessels in the Skagerrak sunk on sight; and Admiral Chester Nimitz, wartime commander-in-chief of the U.S. Pacific Fleet, stated the U.S. Navy had waged unrestricted submarine warfare in the Pacific from the day the U.S. entered the war. Thus although Dönitz was found guilty of waging unrestricted submarine warfare against unarmed neutral shipping by ordering all ships in designated areas in international waters to be sunk without warning, no additional prison time was added to his sentence for this crime.
Dönitz was imprisoned for 10 years in Spandau Prison in what was then West Berlin.