Complex sentence: The Pawnee Agency was established in 1859 for the Pawnee Indians previously assigned to the Otoe Agency, since 1856, and to Council Bluffs Agency prior to that.
Split into simpler sentences: The Pawnee Agency was established in 1859 for the Pawnee. They had previously been assigned to the Otoe Agency since 1856, and to Council Bluffs Agency prior to that.