Complex sentence: It was originally a privately owned facility, used by the St. Louis Agricultural and Mechanical Association for the St. Louis Exposition from 1856 through 1902, though interrupted by the Civil War when the Fairgrounds were used as a Union encampment known as Benton Barracks.
Split into simpler sentences: It was originally a privately owned facility, used by the St. Louis Agricultural and Mechanical Association for the St. Louis Exposition from 1856 through 1902. However, the Civil War interrupted the annual fair when the Fairgrounds were used as a Union encampment known as Benton Barracks.