Complex sentence: He was a brigade and division commander in the IV and VI Corps of the Army of the Potomac, and achieved some success in those roles in the Peninsula Campaign, the Seven Days Battles, and the Battle of Antietam.
Split into simpler sentences: He was a brigade and division commander in the IV and a division commander in VI Corps of the Army of the Potomac. He achieved some success in those roles in the Peninsula Campaign, the Seven Days Battles, and the Battle of Antietam.