Complex sentence: After eighteen years as a captain and almost twenty-four years with the 6th Cavalry Regiment, Whitside was promoted to Major in 1885 and transferred to the 7th Cavalry Regiment, which was then serving in the Dakota Territory at various posts including Fort Meade.
Split into simpler sentences: After eighteen years as a captain and almost twenty-four years with the 6th Cavalry Regiment, Whitside was transferred to the 7th Cavalry Regiment in 1885 and promoted to Major. The 7th was then serving in the Dakota Territory at various posts including Fort Meade.