noted that because of anti-Polish prejudices, in 1939 Fall Weiss served to unite Hitler and the German military in a way that Fall Grün had failed to do in 1938. Hillgruber argued that Hitler's decision to declare war on the United States before he had defeated the Soviet Union was due to Hitler's belief that the United States might quickly defeat Japan, and hence it was better to engage the Americans while they were still involved in a two-front war. Likewise, Hillgruber argued that Hitler's decision to take on the United States in December 1941 was influenced by his belief that the Soviet Union would be defeated by no later than the summer of 1942. In his 1965 book Hitlers Strategie, Hillgruber caused some controversy with his argument that a French attack on the Siegfried Line in the autumn of 1939 would have resulted in a swift German defeat. In 1969 the French historian Albert Merglen expanded on Hillgruber's suggestion by writing a PhD thesis depicting a counter-factual successful French offensive against the Siegfried Line. However, many historians have criticized both Hillgruber and Merglen for ignoring the realities of the time, and for using the advantage of historical hindsight too much in making these judgements. Alternative interpretations Historians have not and do not universally accept Hillgruber's Stufenplan concept. The British historian E.M. Robertson wrote that the Stufenplan concept seemed to explain much of Hitler's foreign policy, but noted that Hitler himself never spoke of having any "stages" or even a plan at all. Moreover, Robertson commented that Hitler's use of the phrase "world power or collapse" in Mein Kampf is ambiguous and can be interpreted in several different ways. However, Robertson went on to note in support of the Stufenplan thesis several speeches Hitler made to his senior officers in late 1938-early 1939, where Hitler did claim to be working out some sort of a master plan in his foreign policy, albeit in a very improvised and flexible way. In a 1970 article, the German historian Martin Broszat wrote that Hitler's decision to invade the Soviet Union was not a "calculated plan to realize his Lebensraum ideas", but that he felt compelled to get out from waiting in the summer of 1940 and proceed to a decisive ending of the war". In response to Broszat, Hillgruber wrote: "In reality, Hitler's decision for a war in the East came in July