Complex sentence: The Iowa District was a region of north-central North America claimed by the United States west of the Mississippi River which from June 28,1834 to July 4,1838 was part first of the Michigan Territory, then of the Wisconsin Territory.
Split into simpler sentences: The Iowa District was a region of north-central North America west of the Mississippi River claimed by the United States. From June 28,1834 to July 4,1838, it was part of the Michigan Territory and, later, the Wisconsin Territory.