In a June 1938 General Staff study, Beck concluded that Germany could defeat Czechoslovakia but that to do so would leave western Germany empty of troops, which could allow the French to seize the Rhineland with little difficulty. Beck maintained that Czechoslovak defences were very formidable, Prague could mobilise at least 38 divisions and at least 30 German divisions would be needed to break through, which required a campaign of at least three weeks. Beck concluded that Hitler's assumptions about a limited war in 1938 were mistaken. In July 1938, upon being shown Beck's 5 May 1938 memorandum opposing "Fall Grün" by Brauchitsch, Hitler called Beck's arguments ""kindische Kräfteberechnungen"" ("childish calculations"). In another memorandum of July 1938, Beck contended that a war against Czechoslovakia, France and Britain would end in Germany's defeat and urged Hitler to postpone his plans for aggression until Germany was strong enough for such a war. In late July 1938, Erich von Manstein, a leading protégé of Beck's, wrote to his mentor urging him to stay at his post and to place his faith in Hitler. On 29 July, Beck wrote a memorandum stating the German Army had the duty to prepare for possible wars with foreign enemies and "for an internal conflict which need only take place in Berlin". The 29 July memo is normally considered to be the start of Beck's efforts to overthrow the Nazi regime.