Complex sentence: In early May, the Germans changed tactics and made local attacks and counter-attacks, which gave the French an opportunity to begin an attack against Fort Douaumont, which was partially occupied, until a German counter-attack reoccupied the fort and took numerous prisoners.
Split into simpler sentences: In early May, the Germans changed tactics and made local attacks and counter-attacks, which gave the French an opportunity to begin an attack against Fort Douaumont. The fort was partially occupied, until a German counter-attack reoccupied the fort and took numerous prisoners.