Complex sentence: Because of Pétain's rotation system for troops, no one served continuously at Verdun for more than a few weeks and most French soldiers fought in the battle; the haunted faces of those being relieved horrified those arriving for the first time.
Split into simpler sentences: Because of Pétain's rotation system for troops, no one served continuously at Verdun for more than a few weeks; thus, most French soldiers fought in the battle. The haunted faces of those being relieved horrified those arriving for the first time.