Overy wrote that blitzkrieg as a "coherent military and economic concept has proven a difficult strategy to defend in light of the evidence". Milward's theory was contrary to Hitler's and German planners' intentions. The Germans, aware of the errors of the First World War, rejected the concept of organising its economy to fight only a short war. Therefore, focus was given to the development of armament in depth for a long war, instead of armament in breadth for a short war. Hitler claimed that relying on surprise alone was "criminal" and that "we have to prepare for a long war along with surprise attack". During the winter of 1939–40, Hitler demobilised many troops from the army to return as skilled workers to factories because the war would be decided by production, not a quick "Panzer operation".