Complex sentence: In 1856 the regiment was engaged in maintaining the peace in the Kansas Territory between the pro-slavery group who fought to make the territory a slave state, and the free-state faction, who bitterly opposed them.
Split into simpler sentences: In 1856 the regiment was engaged in maintaining the peace in the Kansas Territory. Between the pro-slavery group who fought to make the territory a slave state, and free-state faction, who bitterly opposed them.