there is widespread agreement that the war that began in September 1939 was a disaster for Nazi Germany. This was Hitler's own view, clearly expressed at the time. Contrary to the foreign policy axioms laid out in Mein Kampf and frequently repeated by Hitler thereafter, the Third Reich found itself at war with Britain and the British Empire. Contrary to the ‘timetable’ for military expansion that Hitler elaborated in November 1937, the Third Reich found itself involved in a major European war already in 1939, rather than in the years 1943–5, the period that Hitler seemed to believe would be optimal for large-scale imperial expansion. Hitler's own erratic, confused, and increasingly unrealistic conduct of policy in the ten weeks that followed the British and French declarations of war is but one proof of the extreme seriousness of the new international situation brought about by the German invasion of Poland. The Nazi–Soviet Pact was an expedient that did not make good this damage. The invasion had consequences of an irreversibly damaging nature to the interests of the Third Reich (for example, the future intervention of the USA), which Hitler's prior calculations had ruled out as to be avoided at all costs.

What went wrong? I think it is important to put the question in this way because it is quite conceivable that Nazi domination of Europe and of adjacent subcontinents could easily have been wider, longer and even more destructive than it actually was.