The 'Z-Plan' was the German largely unrealised plan, developed via the 'X-Plan' and 'Y-Plan', for the re-equipment and expansion of the German navy ordered by Adolf Hitler (1 March 1939/1940).

Between the Nazi party’s advent to power in 1933 and a time in 1937 the overall strategy of the Reichsmarine (from 21 May 1935 the Kriegsmarine) became centred on Atlantic warfare, with France regarded in all probability as the primary opponent. During the Spanish Civil War (1936/39), Germany was a member of the international Non-Intervention Committee, and Kriegsmarine warships were used in patrol duties. German naval aid to General Francisco Bahamonde y Franco, the nationalist leader, was on a scale far smaller than that provided by the German army and air force: it took the form of about a dozen advisers to aid in the training of Spanish sailors and help in naval administration and development, the patrolling units of the Kriegsmarine’s neutrality patrol detachment remaining neutral.

By this time Hitler was demanding a major battleship force because, he believed, he needed powerful ships for his aggressive policy and to serve as a strong lever in international politics. During the crisis with Austria in May 1938, the German naval leadership came to appreciate for the first time that Hitler believed that a naval confrontation with the UK was possible when he ordered that the battleships F and G (Bismarck and Tirpitz) be finished ahead of schedule, the U-boat construction programme be speeded, the battle-cruisers Scharnhorst and Gneisenau be upgunned to fast battleship standard with their nine 280-mm (11-in) main guns in three triple turrets replaced by six 380-mm (15-in) weapons in three twin turrets, and the design and construction of six 55,450-ton 'H' class super battleships, with a primary armament of eight 406-mm (16-in) guns, be completed with all despatch.

Generaladmiral (from 1 April 1939 Grossadmiral) Erich Raeder, commander-in-chief of the Kriegsmarine, had other ideas about how the defeat of the UK might be encompassed: Appreciating overall British naval superiority, he knew that the only chance of achieving this end was extensive commerce raiding on the high seas to starve the UK of vital imports of food, fuel and raw materials and so being about a British capitulation. Raeder’s concept demanded more Panzerschiffe (pocket battleships), long-range light cruisers, Spähkreuzer large destroyers, and U-boats. In Raeder’s thinking a battle fleet, no matter how potent, would be as useless as it had been in World War I because the UK would again blockade the North Sea.

During internal discussions within the Kriegsmarine, this original concept was modified in order to accommodate Hitler’s battleship ambitions, and this created the 'Z-Plan'. In its first form of 27 January 1939, this was rejected by the Hitler, who countered Raeder’s opposition by threatening to appoint a civilian overlord for the whole programme of naval construction.

As finally agreed on 1 March 1939, the 'Z-Plan' comprised the following ships, which would remain in commission until 1948 (the figures in parentheses give the numbers of ships to be commissioned, or under construction, in 1939): four aircraft carriers (two), eight battleships (four), five battle-cruisers (two); eight heavy cruisers (eight including the five pocket battleships), 13 light cruisers (nine), 22 large destroyers (zero); 68 destroyers (30), 90 torpedo boats (36), 249 U-boats (129), 302 small fighting vessels (187), 10 mineships and minelayers (three), and 909 auxiliaries (123).

The plan overruled the contention of the U-boat faction, led by Kommodore Karl Dönitz, commanding the U-boat arm, that the real 'equaliser' in any war with the UK would be the U-boat, and therefore that U-boat production should be accorded he highest priority.

As promulgated in March 1939, therefore, the 'Z-Plan' was designed to create a Kriegsmarine capable of challenging the Royal Navy on a global basis through the creation by 1944 of a German navy with 10 battleships, four aircraft carriers, three battle-cruisers, eight heavy cruisers, 44 light cruisers, 68 destroyers and 249 U-boats.

These vessels were to constitute the strength of four primary formations: one home fleet, one raiding force and two attack forces. Designed to contest naval supremacy with the British Home Fleet, the German home fleet was to have four super-battleships, two heavy cruisers and several flotillas of destroyers. Designed to take the war to British merchant shipping, the raiding force was to comprise three pocket battleships, five heavy cruisers, five light cruisers and 190 U-boats. Designed to support either of the two forces above, the two attack forces were each to comprise one aircraft carrier, three fast battleships, two cruisers and several flotillas of destroyers.

In this period shortly before the start of World War II, therefore, the German naval effort was effectively polarised into two camps, one politically motived and the other professionally mandated. Early in his early political career, Hitler had criticised the Imperial German navy’s concept of a large battle fleet as being only a source of communist-inspired mutinies, but now ordered a major fleet as a political rather than purely naval instrument. Believing in the strategy of long-range surface commerce raiding, on the other hand, the Kriegsmarine had to live with a naval construction programme comprising not only a powerful surface fleet, but also a medium-sized U-boat arm.

It is at best questionable whether a task force composed of a carrier, 'H' class super battleships, 'P' class cruisers, large destroyers and fleet destroyers could have operated successfully in the Atlantic without any nearby shore base and therefore reliant on support ships for supply and repair.

At the industrial level, the acceleration of this ambitious building programme created an acute shortage of materials (especially high-grade steel) and skilled dockyard personnel. In addition there arose a fierce struggle and deteriorating relationships among the three services as each sought to win priority treatment for itself (the army was in the throes of expanding its armoured forces, and the air force was rapidly enlarging its bomber force). The creation of a viable naval air arm was also being jeopardised by Generalfeldmarschall Hermann Göring’s insistence that 'everything with wings belongs to me'.

In 1939 all Germany’s in-service capital ships had suffered political interference in their design characteristics, only two genuine battleships were in the final stages of fitting out, and the construction of the first carrier was being delayed by technical problems and the lack of interest shown by the Luftwaffe.

After the end of World War I, the German armed forces had been subjected to limitations imposed in 1919 by the Treaty of Versailles. The German navy had thereby been limited to six Panzerschiffe armoured warships, six cruisers, 12 destroyers and 12 torpedo-boats. With the scuttling of the majority of the High Seas Fleet while interned at Scapa Flow in 1919, new construction was needed and the first major ship to be built by Germany after the end of World War I was the light cruiser Emden. This was followed by a further three 'K' class light cruisers (Königsberg, Karlsruhe and Köln), and a further two ships that were modified 'K' class units (Leipzig and Nürnberg).

The treaty also stipulated that Germany could replace its armoured ships as needed, but only with vessels of a displacement no greater than 10,000 tons. Thus a new Panzerschiff concept was created: this was designed as a Diesel-engined vessel with considerable range and intended primarily to prey on merchant shipping: as such the ships were to be 'stronger than faster enemies' (cruisers) and 'faster than stronger enemies' (capital ships).

The first of the new vessels was Deutschland, with a primary armament of six 280-mm (11-in) guns, a speed of 28 kt and a range of 10,000 nm (11,500 miles (18500 km) at 20 kt, and there followed another two such pocket battleships as Admiral Scheer and Admiral Graf Spee.

The Royal Navy had also considered this design concept with its battle-cruiser class designs. The principal constraint on speed was the net power/gross tonnage ratio. The British compromise was to reduce the weight of armour that the battle-cruiser carried, thus reducing the deadweight displacement of the ship, but even so the maximum speed requirement of the battle-cruiser meant that the British designs were based on a larger hull capable of accommodating a more powerful propulsion plant. This resulted largely from the fact that British propulsion plant designs of the period were less technically sophisticated than those of the Germans.

When the Germans came to design battle-cruisers, the high speed requirement was met with smaller propulsion plants of greater power/deadweight ratios. Therefore the German pocket battleship hulls could be dimensionally smaller than those of the British but with a greater thickness and percentage of armour weight.

In 1933 Adolf Hitler became the chancellor of Germany, and very soon withdrew from the stipulations of the Treaty of Versailles and began the systematic rebuilding of the German armed forces. The prestige brought by the Panzerschiffe concept in its original form led to orders for a pair of improved vessels, the Panzerschiffe D and E, which became Scharnhorst and Gneisenau: these were bigger than their predecessors, with nine instead of six guns. At the same time, studies were made into the construction of two even larger vessels. These were initially to have been of the Panzerschiffe type with a main armament of 330-mm (13-in) guns, but with the improvements to the French fleet at the time, the new ships were redesigned to full Schlachtschiffe battleship standard.

At this time, it was decided to embark on a large-scale revitalisation and enlargement of the German navy, and it was thus that the 'Z-Plan' came to be evolved. Within the German navy at this time there were two widely separate schools of thought about the course that the new plan should follow: one school desired the creation of a large surface battle fleet capable of taking on the most powerful prospective opponents, namely France and the UK, while the other school believed in the creation of a large force of U-boats and medium-sized warships such as the Panzerschiffe optimised for the destruction of an opponent’s merchant shipping. The advocates of the powerful surface navy pointed out that in order to carry out commerce raiding in the Atlantic Ocean, German ships would have to pass through the North Sea, which was likely to be filled with British battleships, and it was their thinking which prevailed.

As decided, therefore, the 'Z-Plan' would have seen the completion of the two battleships currently under construction (Bismarck and Tirpitz) to an interim design, as well as three heavy cruisers (Admiral Hipper, Blücher and Prinz Eugen), plus a further two launched in 1939, before the major construction work began. The plan was then to have completed by 1945 of the majority of a force of four aircraft carriers, six 'H' class battleships, three 'O' class battle-cruisers, 12 'Kreuzer P' class Panzerschiffe, two more 'Hipper' class heavy cruisers (Seydlitz and Lützow), four 'M' class light cruisers, two 'Improved M' class light cruisers, and six Spähkreuzer large destroyers.

Work on the new programme started with the aircraft carrier Graf Zeppelin in 1936, with a second planned to begin in 1938. In mid-1939, following the launch of Bismarck and Tirpitz, the keels of the first three improved battleships were laid, while orders were placed for the modified versions of the Messerschmitt Bf 109 fighter and Junkers Ju 87 dive-bombers for carrierborne service.

With the outbreak of World War II, however, it was decided that these large and expensive construction projects required too much of the materials vital to keep the army and Luftwaffe up and running. Work on the battleships was halted, therefore, and from about 1940 the materials were diverted to other tacks including the construction of U-boats.