Complex sentence: The men of the 55th, according to the Gettysburg Commission, had advanced the farthest during Pickett's Charge, also known as the high-water mark of the Confederacy.
Split into simpler sentences: The men of the 55th, according to the Gettysburg Commission, had advanced the farthest during Pickett's Charge. Also know as the high-water mark of the Confederacy.