The area that became the Illinois Territory was earlier known as Illinois Country and was under French control, first as part of French Canada and then as part of Louisiana. The British gained authority over the region with the 1763 Treaty of Paris marking the end of the French and Indian War. During the American Revolutionary War, General George Rogers Clark (then a Colonel) took possession of the entire Illinois Country for Virginia, which formed the "county of Illinois" to exercise nominal governance over the area. Virginia ceded nearly all of its claims to land north of the Ohio River in order to satisfy objections of land-locked states.
What was to become Illinois Territory was a portion of the larger Northwest Territory from 13 July 1787 until 4 July 1800, when the Indiana Territory was formed as Ohio prepared to become a state. On 3 February 1809, the 10th United States Congress passed legislation establishing Illinois Territory, after Congress received petitions from residents in the far western areas complaining of the difficulties of participating in territorial affairs.
The Illinois Territory originally included areas that became the states of Illinois, Wisconsin, the eastern portion of Minnesota, and the western portion of the upper peninsula of Michigan. As Illinois was preparing to become a state, the remaining area of the territory was attached to the Michigan Territory and ceased using the name Illinois Territory.
The original boundaries of the Territory were defined as follows:
“...all that part of the Indiana Territory which lies west of the Wabash river, and a direct line drawn from the said Wabash river and Post Vincennes, due north to the territorial line between the United States and Canada...”
Kaskaskia was the territorial capital.