When World War II broke out in September 1939, Plan Z was shelved in favour of a crash building programme for submarines (U-boats) instead of capital surface warships, and land and air forces were given priority of strategic resources.

The Commander-in-Chief of the Kriegsmarine (as for all branches of the armed forces during the period of absolute Nazi power) was Adolf Hitler, who exercised his authority through the Oberkommando der Marine ('High Command of the Navy').

Wolfpacks were rapidly assembled groups of submarines which attacked British convoys during the first half of the Battle of the Atlantic, but this tactic was largely abandoned by May 1943, when U-boat losses mounted.

[4] Even before the Nazi seizure of power on 30 January 1933 the German government decided on 15 November 1932 to launch a prohibited naval re-armament program that included U-boats, airplanes, and an aircraft carrier.

The building of the Deutschland caused consternation among the French and the British as they had expected that the restrictions of the Treaty of Versailles would limit the replacement of the pre-dreadnought battleships to coastal defence ships, suitable only for defensive warfare.

By using innovative construction techniques, the Germans had built a heavy ship suitable for offensive warfare on the high seas while still abiding by the letter of the treaty.

Submarine attacks on Britain's vital maritime supply routes (Battle of the Atlantic) started immediately at the outbreak of war, although they were hampered by the lack of well placed ports from which to operate.

[7] In April 1940, the German Navy was heavily involved in the invasion of Norway, where it suffered significant losses, which included the heavy cruiser Blücher sunk by artillery and torpedoes from Norwegian shore batteries at the Oscarsborg Fortress in the Oslofjord.

The losses in the Norwegian Campaign left only a handful of undamaged heavy ships available for the planned, but never executed, invasion of the United Kingdom (Operation Sea Lion) in the summer of 1940.

The Fall of France and the conquest of Norway gave German submarines greatly improved access to British shipping routes in the Atlantic.

At first, British convoys lacked escorts that were adequate either in numbers or equipment and, as a result, the submarines had much success for few losses (this period was dubbed the First Happy Time by the Germans).

Italy entered the war in June 1940, and the Battle of the Mediterranean began: from September 1941 to May 1944 some 62 German submarines were transferred there, sneaking past the British naval base at Gibraltar.

[9] During 1941, the Kriegsmarine and the United States Navy became de facto belligerents, although war was not formally declared, leading to the sinking of the USS Reuben James.

The Japanese attack on Pearl Harbor and the subsequent German declaration of war against the United States in December 1941 led to another phase of the Battle of the Atlantic.

It was a tactical victory for the Kriegsmarine and a blow to British morale, but the withdrawal removed the possibility of attacking allied convoys in the Atlantic with heavy surface ships.

With the German attack on the Soviet Union in June 1941 Britain started to send Arctic convoys with military goods around Norway to support their new ally.

After December 1943 when Scharnhorst had been sunk in an attack on an Arctic convoy in the Battle of North Cape by HMS Duke of York, most German surface ships in bases at the Atlantic were blockaded in, or close to, their ports as a fleet in being, for fear of losing them in action and to tie up British naval forces.

From late 1944 until the end of the war, the surviving surface fleet of the Kriegsmarine (heavy cruisers: Admiral Scheer, Lützow, Admiral Hipper, Prinz Eugen, light cruisers: Nürnberg, Köln, Emden) was heavily engaged in providing artillery support to the retreating German land forces along the Baltic coast and in ferrying civilian refugees to the western Baltic Sea parts of Germany (Mecklenburg, Schleswig-Holstein) in large rescue operations.

[19] On 22 July, Kawelmacher sent a telegram to the German Navy's Baltic Command in Kiel, which stated that he wanted 100 SS and fifty Schutzpolizei (protective police) men sent to Liepāja for "quick implementation Jewish problem".

"[21] Kawelmacher telegram on 27 July 1941 read: "Jewish problem Libau largely solved by execution of about 1,100 male Jews by Riga SS commando on 24 and 25.7.

"[20] In September 1939, U-boat commander Fritz-Julius Lemp of U-30 sank SS Athenia (1922) after mistaking it for a legitimate military target, resulting in the deaths of 117 civilians.

After the war, the German surface ships that remained afloat (only the cruisers Prinz Eugen and Nürnberg, and a dozen destroyers were operational) were divided among the victors by the Tripartite Naval Commission.

Some (like the unfinished aircraft carrier Graf Zeppelin) were used for target practice with conventional weapons, while others (mostly destroyers and torpedo boats) were put into the service of Allied navies that lacked surface ships after the war.

The training barque SSS Horst Wessel was recommissioned USCGC Eagle and remains in active service, assigned to the United States Coast Guard Academy.

The Bismarck was sunk on her first sortie into the Atlantic in 1941 (Operation Rheinübung) although she did sink the battlecruiser Hood and severely damaged the battleship Prince of Wales, while the Tirpitz was based in Norwegian ports during most of the war as a fleet in being, tying up Allied naval forces, and subject to a number of attacks by British aircraft and submarines.

During the war, some merchant ships were converted into "auxiliary cruisers" and nine were used as commerce raiders sailing under false flags to avoid detection, and operated in all oceans with considerable effect.

The Kriegsmarine employed hundreds of auxiliary Vorpostenboote during the war, mostly civilian ships that were drafted and fitted with military equipment, for use in coastal operations.

[36] Five coastal groups (Küstenfliegergruppen) with reconnaissance aircraft, torpedo bombers, Minensuch aerial minesweepers, and air-sea rescue seaplanes supported the Kriegsmarine, although with lesser resources as the war progressed.

[38] At the beginning of World War II, on 1 September 1939, the Marine Stoßtrupp Kompanie (Naval Shock Troop Company) landed in Danzig from the old battleship Schleswig-Holstein for conquering a Polish bastion at Westerplatte.

With the invasion of Normandy in June 1944 and the Soviet advance from the summer of 1944 the Kriegsmarine started to form regiments and divisions for the battles on land with superfluous personnel.