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Plan Z was the name given to the planned re-equipment and expansion of the Kriegsmarine from 1935 onwards.

Following the end of the First World War, the German armed forces became subject to the stipulations of the Treaty of Versailles. For the navy, this meant it was restricted to six armoured warships ("panzerschiffe"), six cruisers, twelve destroyers and twelve torpedo-boats. With the scuttling of the majority of the High Seas Fleet at Scapa Flow, new construction was needed. The first major ship to be built after the war was the light cruiser Emden. This was followed by a further three light cruisers of the "K class", Königsberg, Karlsruhe and Köln, and a further two ships that were modified versions of the "K class", Leipzig and Nürnberg. The Treaty also stipulated that Germany could replace its armoured ships as needed, but with vessels that were not more than 10,000 tons. So, the panzerschiff concept was thought up - this was designed primarily as an aggressor towards merchant shipping, with the priviso that it be "stronger than faster enemies" (cruisers) and "faster than stronger enemies" (battleships). This led to the Deutschland, a ship with six 11-inch guns and a speed of 28 knots. Two further units, Admiral Scheer and Admiral Graf Spee followed. These were called "pocket battleships" in the outside world.

In 1933, Adolf Hitler became the Chancellor of Germany. He withdrew from the stipulations of the Treaty of Versailles and began the systematic re-building of the armed forces. The prestige brought by the panzerschiffe led to two improved vessels, Panzerschiffe D & E to be ordered - these became the Scharnhorst and Gneisenau, which were bigger than their predecessors, with nine instead of six guns. At the same time, studies were made into the construction of two even bigger vessels. These were initially going to be panzerschiffe with 13" guns, but with the improvements to the French fleet at the time, the new ships were redesigned as full schlachtschiffe (battleships). At this time, it was decided to embark on a large scale re-building of the German Navy, and so Plan Z was evolved.

Within the Kriegsmarine, two bodies of opinion were massed as to the direction of the re-equipment of the Navy:

• A large battle fleet capable of taking on the most powerful prospective opponents (Britain and France).
• A large force of U-Boats and medium sized warships such as the panzerschiffe for destruction of the enemys commercial shipping.

However, it was 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. So, the large fleet option was the one chosen.

The plan as it came about would have seen the completion of the two battleships 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 the majority finished by 1945:

• Four aircraft carriers
• Six H Class battleships
• Three "O Class" battlecruisers
• Twelve "Kreuzer P Class" Panzerschiffe
• Two Hipper Class heavy cruisers (Seydlitz, and Lützow)
• Four "M Class" light cruisers
• Two "Improved M Class" light cruisers
• Six " Spähkreuzer Class" large destroyers

The first work saw construction begin on the aircraft carrier Graf Zeppelin in 1936, with a second planned to begin in 1938. In mid 1939, following the launch of both Bismarck and Tirpitz, the keels of the first three improved battleships were laid, while orders were placed for the modified Messerschmitt Bf 109 fighters and Junkers Ju87 dive bombers for the aircraft carriers. However, with the outbreak of the Second World War, it was decided that the large and expensive construction projects required too much of the materials vital to keep the army and Luftwaffe up and running. As a consequence, work on the battleships was halted, and the materials were diverted to the construction of U-Boats.