These considerations (the bare survival of the German people), which all pointed to the need for the creation of some sort of central government, took shape and form when I was joined by Graf Schwerin-Krosigk. In addition to discharging his duties as Foreign Minister and Minister of Finance, he formed the temporary government we needed and presided over the activities of its cabinet. Though restricted in his choice to men in northern Germany, he nonetheless succeeded in forming a workmanlike cabinet of experts. The picture of the military situation as a whole showed clearly that the war was lost. As there was also no possibility of effecting any improvement in Germany's overall position by political means, the only conclusion to which I, as head of state, could come was that the war must be brought to an end as quickly as possible in order to prevent further bloodshed.— Karl Dönitz, Ten Years and Twenty Days
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 SMS Breslau in the Mediterranean Sea. In August 1914, the Breslau and the battlecruiser SMS Goeben 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. When the Midilli put into dock for repairs, he was temporarily assigned as airfield commander at the Dardanelles. From there, he requested a transfer to the submarine forces, which became effective in October 1916. He served as watch officer on U-39, and from February 1917 onward as commander of UC-25. On 5 September 1917, he became commander of UB-68, operating in the Mediterranean. On 4 October, after suffering technical difficulties, this boat was sunk by British forces and Dönitz was taken prisoner on the island of Malta.
On 27 May 1916, Dönitz married a nurse named Ingeborg Weber (1894–1962), the daughter of a German general Erich Weber (1860–1933). They had three children whom they raised as Protestant Christians: daughter Ursula (1917–1990) and sons Klaus (1920–1944) and Peter (1922–1943).
During the interwar period, he continued his naval career in the naval arm of the Weimar Republic's armed forces. On 10 January 1921, he became a Kapitänleutnant (lieutenant) in the new German navy (Vorläufige Reichsmarine). Dönitz commanded torpedo boats, becoming a Korvettenkapitän (lieutenant-commander) on 1 November 1928.
Dönitz revived the World War I idea of grouping several submarines together into a "wolfpack" to overwhelm a merchant convoy's defensive escorts. Implementation of wolfpacks had been difficult in World War I owing to the limitations of available radios. In the interwar years, Germany had developed ultrahigh frequency transmitters which it was hoped would make their radio communication unjammable, while the Enigma cipher machine was believed to have made communications secure. Dönitz also adopted and claimed credit for Wilhelm Marschall's 1922 idea of attacking convoys using surface or very-near-the-surface night attacks. This tactic had the added advantage of making a submarine undetectable by sonar.
On 1 September 1933, he became a Fregattenkapitän (commander) and, in 1934, was put in command of the cruiser Emden, the ship on which cadets and midshipmen took a year-long world cruise in preparation for a Future officer's commission.
Dönitz's second book, Mein wechselvolles Leben (My Ever-Changing Life) is less known, perhaps because it deals with the events of his life before 1934. This book was first published in 1968, and a new edition was released in 1998 with the revised title Mein soldatisches Leben (My Martial Life).
Throughout 1935 and 1936, Dönitz had misgivings regarding submarines due to German overestimation of the capabilities of British ASDIC intelligence. In reality, ASDIC could detect only one submarine in 10 during exercises. In the words of Alan Hotham, British Director of Naval Intelligence, ASDIC was a "huge bluff".
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."
In 1937, Karl Dönitz's daughter Ursula married U-boat commander Günther Hessler.
Among the war-crimes charges, Dönitz was accused of waging unrestricted submarine warfare for issuing War Order No. 154 in 1939, and another similar order after the Laconia incident in 1942, not to rescue survivors from ships attacked by submarine. By issuing these two orders, he was found guilty of causing Germany to be in breach of the Second London Naval Treaty of 1936. However, as evidence of similar conduct by the Allies was presented at his trial, and with the help of his Lawyer Otto Kranzbühler, his sentence was not assessed on the grounds of this breach of international law.
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.
By 1941, the delivery of new Type VIIs had improved to the point where operations were having a real effect on the British wartime economy. Although production of merchant ships shot up in response, improved torpedoes, better U-boats, and much better operational planning led to increasing numbers of "kills". On 11 December 1941, following Hitler's declaration of war on the United States, Dönitz immediately planned for implementation of Operation Drumbeat (Unternehmen Paukenschlag). This targeted shipping along the East Coast of the United States. Carried out the next month with only nine U-boats (all the larger Type IX), it had dramatic and far-reaching results. The U.S. Navy was entirely unprepared for antisubmarine warfare despite having had two years of British experience to draw from, and committed every imaginable mistake. Shipping losses, which had appeared to be coming under control as the Royal Navy and Royal Canadian Navy gradually adapted to the new challenge, skyrocketed.
By the end of 1942, the production of Type VII U-boats had increased to the point where Dönitz was finally able to conduct mass attacks by groups of submarines, a tactic he called Rudel (group or pack) and became known as "wolfpack" in English. Allied shipping losses shot up tremendously, and serious concern existed for a while about the state of British fuel supplies.
Both of Dönitz's sons were killed during the Second World War. The younger, Peter, was killed on 19 May 1943, when U-954 was sunk in the North Atlantic with all hands. After this loss, the elder son, Klaus, was allowed to leave combat duty and began studying to be a naval Doctor. Klaus was killed on 13 May 1944 while taking part in an action contrary to standing orders prohibiting his involvement in any combat role. He persuaded his friends to let him go on the torpedo boat S-141 for a raid on Selsey on his 24th birthday. The boat was sunk by the French destroyer La Combattante and Klaus died, though six others were rescued.
On German Heroes' Day (12 March) of 1944, Dönitz declared that without Adolf Hitler, Germany would be beset by "the poison of Jewry", and the country destroyed for lack of National Socialism, which, as Dönitz declared, met an uncompromising ideology with defiance. At the Nuremberg trials, Dönitz claimed the statement about the "poison of Jewry" was regarding "the endurance, the power to endure, of the people, as it was composed, could be better preserved than if there were Jewish elements in the nation". Initially he claimed, "I could imagine that it would be very difficult for the population in the towns to hold out under the strain of heavy bombing attacks if such an influence were allowed to work."
A day later, Dönitz sent Friedeburg to U.S. General Dwight D. Eisenhower's headquarters in Rheims, France, to negotiate a surrender to the Allies. The Chief of Staff of OKW, Generaloberst (Colonel-General) Alfred Jodl, arrived a day later. Dönitz had instructed them to draw out the negotiations for as long as possible so that German troops and refugees could surrender to the Western powers, but when Eisenhower let it be known he would not tolerate their stalling, Dönitz authorised Jodl to sign the instrument of unconditional surrender at 1:30 on the morning of 7 May. Just over an hour later, Jodl signed the documents. The surrender documents included the phrase, "All forces under German control to cease active operations at 23:01 hours Central European Time on 8 May 1945." At Stalin's insistence, on 8 May, shortly before midnight, (Generalfeldmarschall) Wilhelm Keitel repeated the signing in Berlin at Marshal Georgiy Zhukov's headquarters, with General Carl Spaatz of the USAAF present as Eisenhower's representative. At the time specified, World War II in Europe ended.
Dönitz was released on 1 October 1956 and retired to the small village of Aumühle in Schleswig-Holstein in northern West Germany. There, he worked on two books. His memoirs, Zehn Jahre, Zwanzig Tage (Memoirs: Ten Years and Twenty Days), were released in Germany in 1958 and became available in an English translation the following year. This book recounted Dönitz's experiences as U-boat commander (10 years) and President of Germany (20 days). In it, Dönitz explains the Nazi regime as a product of its time, but argues he was not a Politician and thus not morally responsible for many of the regime's crimes. He likewise criticizes dictatorship as a fundamentally flawed form of government and blames it for many of the Nazi era's failings.
In 1973, he appeared in the Thames Television production The World at War, in one of his few television appearances.
Dönitz lived out the rest of his life in relative obscurity in Aumühle, occasionally corresponding with Collectors of German naval history, and died there of a heart attack on 24 December 1980. As the last German officer with the rank of Großadmiral (grand admiral), he was honoured by many former servicemen and foreign naval officers who came to pay their respects at his funeral on 6 January 1981. He was buried in Waldfriedhof Cemetery in Aumühle without military honours, and Soldiers were not allowed to wear uniforms to the funeral. However, a number of German naval officers disobeyed this order and were joined by members of the Royal Navy, such as the senior chaplain, the Rev. Dr. John Cameron, in full dress uniform. Also in attendance were over 100 holders of the Knight's Cross of the Iron Cross.
Dönitz was, for nearly seven decades, the only head of state to be convicted by an international tribunal until the conviction of Liberia's Charles Taylor in April 2012.
At the time, many – including Admiral Erich Raeder – felt such talk marked Dönitz as a weakling. Dönitz was alone among senior naval officers, including some former submariners, in believing in a new submarine war on trade. Dönitz and Raeder argued constantly over funding priorities within the Navy, while at the same time competing with Hitler's friends, such as Hermann Göring, who received greater attention at this time.
Dönitz disputed the propriety of his trial at Nuremberg, commenting on count (2) "One of the 'accusations' that made me guilty during this trial was that I met and planned the course of the war with Hitler; now I ask them in heaven’s name, how could an admiral do otherwise with his country's head of state in a time of war?" Over 100 senior Allied officers also sent letters to Dönitz conveying their disappointment over the fairness and verdict of his trial.