Complex sentence: One other Arkansas regiment was also designated as the 39th Arkansas Infantry; the regiment commanded by Colonels Hart, McNeill, and Rogan, was originally designated as the 39th Arkansas Infantry, but was later redesignated as the 30th Arkansas Infantry Regiment.
Split into simpler sentences: One other Arkansas regiment was designated as the 39th Arkansas Infantry; that regiment being successively commanded by Colonels Hart, McNeill, and Rogan. It was originally designated as the 39th Arkansas, but later redesignated as the 30th Arkansas Infantry Regiment.