The "Kriegsmarine" was developed during the German naval rearmament of the 1930s. The Treaty of Versailles had limited the size of the German navy after World War I. It also prevented Germany from building submarines. But by 1934 Germany had secretly most of the parts they needed to construct submarines (called U-boats). In early 1935 Hitler ordered the U-boats to be built. In January 1939 Plan Z was ordered, calling for the construction of many naval vessels. The ships of the "Kriegsmarine" fought during the Spanish Civil War and World War II. The commander of the "Kriegsmarine" was Karl Dönitz.