The Northwest Ordinance of 1787, passed just before the U.S. Constitution was ratified, prohibited slavery in the Northwest Territory. The southern boundary of the territory was the Ohio River. This was regarded as a westward extension of the Mason–Dixon line. The territory was generally settled by New Englanders and American Revolutionary War veterans who were granted land there. The states created from the territory – Ohio (1803), Indiana (1816), Illinois (1818), Michigan (1837), Iowa (1846), Wisconsin (1848), and Minnesota (1858) – were all free states.