Complex sentence: A week later the Germans began the Battle of Verdun and the costly French defence of the Meuse Heights, eventually reducing the French contribution to the Somme offensive to of the Sixth Army and divisions in the Fourth Army.
Split into simpler sentences: A week later the Germans began the Battle of Verdun. The costly defence of Verdun forced the French army to commit divisions intended for the Somme offensive, eventually reducing the French contribution to in the Sixth Army, against divisions in the Fourth Army.