 Throughout the first half of the 19th century , the northeastern portion of the state was a part of the Northwest Territory , formed in 1787 . After Ohio 's statehood the area became part of the new Illinois Territory in 1809 . After Illinois ' statehood the area was incorporated into the Michigan Territory in 1818 and later became part of the Wisconsin Territory in 1836 . The western and southern areas of the state were not formally organized until 1838 , when they became part of the Iowa Territory . 
