Complex sentence: The chief revisionist historians, who generally favor the Copperheads, are Richard O. Curry and Frank L. Klement, who devoted most of his career to debunking the idea that the Copperheads represented danger to the Union.
Split into simpler sentences: The chief revisionist historians, who generally favor the Copperheads, are Richard O. Curry and Frank L. Klement. Klement devoted most of his career to debunking the idea that the Copperheads represented a danger to the Union.