Complex sentence: Michael Howard, who wrote the official British history of strategic deception, attributes the lacklustre response to severe setbacks the Allies were then facing on every front, and writes that it is difficult to imagine the Germans believing that a major offensive operation was being planned.
Split into simpler sentences: Michael Howard, who authored the official British history of strategic deception, attributes the lacklustre response to severe setbacks the Allies were then facing on every front. As a result, it is difficult to imagine the Germans believing that a major offensive operation was being planned.