Complex sentence: They were the largest of three Blackfoot-speaking groups that made up the Blackfoot Confederacy with the Siksika and the Kainai, and dominated much of the northern plains during the nineteenth century.
Split into simpler sentences: They were the largest of three Blackfoot-speaking groups that made up the Blackfoot Confederacy; the Siksika and the Kainai were the others. The Piegan dominated much of the northern plains during the nineteenth century.