What geographical factors led to the early success of the German blitz?
First, I assume that you are not really asking about the Blitz. Instead, I think that you are asking about the German tactic of “blitzkrieg.” The Blitz was the early German bombing campaign against England, and London in particular. Geography had very little to do with the early success of that part of the war.
Blitzkrieg is a word that means “lightning war.” It was the German tactic that was developed for use in this war and which devastated the defenses of the French and English and, at least initially, the Soviet Union. Geography helped blitzkrieg succeed because the places that it was originally used were generally flat and open. This was important for blitzkrieg because of the fact that blitzkrieg relies heavily on tanks and on close air support. Neither of these would be easy to use if the terrain were very mountainous or if it were forested. Therefore, the German strategy of blitzkrieg depended upon having geography that was relatively flat and open.