Complex sentence: When Wisconsin was admitted into the union as a state on May 29,1848, St. Croix County was further divided, with the territory west of the state border re-organized a year later as part of Minnesota Territory, forming Washington, Ramsey and Benton Counties.
Split into simpler sentences: When Wisconsin was admitted into union as a State on May 29,1848, St. Croix County was divided into two. The portion of St. Croix County of the residual Wisconsin Territory was re-organized a year later as part of Minnesota Territory, forming Washington, Ramsey and Benton Counties.