Complex sentence: He was wounded in the thigh and left the battle (his senior brigadier Roger A. Pryor taking over) without which his division began to falter and eventually succumb to Union flank attacks that routed them from their position.
Split into simpler sentences: He was wounded in the thigh and left the battle. After Anderson's departure, his division began to falter and eventually succumb to Union flank attacks that routed them from their position.