Complex sentence: The beginning of the end of the Scourge came at the Battle of Verdun when the battle began, air superiority created by the Fokkers meant that German preparations had mostly been concealed from French aerial reconnaissance.
Split into simpler sentences: The beginning of the end of the Scourge came at the Battle of Verdun. When the battle began, air superiority created by the Fokkers meant that German preparations had mostly been concealed from French aerial reconnaissance.