Complex sentence: The Allies and the Germans attempted to take more ground after the "open" northern flank had disappeared, the Franco-British attacks towards Lille in October being followed up by attempts to advance between the BEF and the Belgian army by a new French Eighth Army.
Split into simpler sentences: The Allies and the Germans attempted to take more ground after the "open" northern flank had disappeared. The Franco-British attacks towards Lille in October were followed up by attempts to advance between the BEF and the Belgian army by a new French Eighth Army.