Germany was forbidden a military air force by the 1919 Treaty of Versailles, and therefore air crew were trained by means of civilian and sport flying. Following a 1923 memorandum, the Deutsche Luft Hansa airline developed designs for aircraft such as the Junkers Ju 52, which could carry passengers and freight, but be readily adapted into bombers. In 1926, the secret Lipetsk fighter-pilot school began operating. Erhard Milch organised rapid expansion, and following the 1933 Nazi seizure of power, his subordinate Robert Knauss formulated a deterrence theory incorporating Douhet's ideas and Tirpitz's "risk theory", which proposed a fleet of heavy bombers to deter a preventive attack by France and Poland before Germany could fully rearm. A 1933–34 war game indicated a need for fighters and anti-aircraft protection as well as bombers. On 1 March 1935, the Luftwaffe was formally announced, with Walther Wever as Chief of Staff. The 1935 Luftwaffe doctrine for "Conduct of the Air War" (Die Luftkriegführung) set air power within the overall military strategy, with critical tasks of attaining (local and temporary) air superiority and providing battlefield support for army and naval forces. Strategic bombing of industries and transport could be decisive longer term options, dependent on opportunity or preparations by the army and navy, to overcome a stalemate or used when only destruction of the enemy's economy would be conclusive. The list excluded bombing civilians to destroy homes or undermine morale, as that was considered a waste of strategic effort, but the doctrine allowed revenge attacks if German civilians were bombed. A revised edition was issued in 1940, and the continuing central principle of Luftwaffe doctrine was that destruction of enemy armed forces was of primary importance.