Between 1933 and 1945, the organization of the Luftwaffe underwent several changes. Originally, the German military high command, for their air warfare forces, decided to use an organizational structure similar to the army and navy, treating the aviation branch as a strategic weapon of war. Later on, during the period of rapid rearmament, the Luftwaffe was organized more in a geographical fashion. Under the terms of the Treaty of Versailles (1919), Germany was prohibited from having an air force, with the former German Empire's Luftstreitkräfte disbandment in 1920. German pilots were secretly trained for military aviation, first in the Soviet Union during the late 1920s, and then in Germany in the early 1930s. In Germany, the training was done under the guise of the German Air Sports Association ( (DLV)) at the Central Commercial Pilots School ( (ZVF)). Following its 15 May 1933 formation in secret, the formation of the German air arm was openly announced in February 1935, with Reichsmarschall Hermann Göring as its Commander-in-Chief (), in blatant defiance of the Versailles Treaty. Initial plans were for long-term growth of the Luftwaffe over a period of five years with the intention of using the Luftwaffe as a strategic force. These plans were changed several times, especially after the June 1936 death of Walter Wever and the succession of Ernst Udet. The focus and role of the Luftwaffe became one of ground support for the German Army during its Lightning War () campaigns. Göring, using his political capital, was able to get significant resources allocated to the Luftwaffe, more so than the army () or the navy (); all three forces existing within the combined Wehrmacht German armed forces of the Reich. This made the Luftwaffe one of the most powerful air forces in Europe during its initial years. Partly due to its ground support role, the Luftwaffe was reorganized in a fashion similar to the army units, with one unit controlling a specific area. Each Luftwaffe unit was self-contained and had complete control over all aspects of Luftwaffe forces in that area. Before becoming head of the Luftwaffe, Göring was Interior Minister of Prussia. In this position he had formed his own army, starting from a 400 men police department to regiment size. When Göring took over the Luftwaffe, he brought the regiment along with him to the Luftwaffe and created his own ground forces in the form of