The Territory of Illinois was an organized incorporated territory of the United States that existed from March 1, 1809, until December 3, 1818, when the southern portion of the territory was admitted to the Union

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 as the State of Illinois. The area was earlier known as "Illinois Country

Illinois Country

The Illinois Country , also known as Upper Louisiana, was a region in what is now the Midwestern United States that was explored and settled by the French during the 17th and 18th centuries. The terms referred to the entire Upper Mississippi River watershed, though settlement was concentrated in...

" while under French control, first as part of French Canada

French Canada

French Canada, also known as "Lower Canada", is a term to distinguish the French Canadian population of Canada from English Canada.-Definition:...

 and then as part of Louisiana

Louisiana (New France)

Louisiana or French Louisiana was an administrative district of New France. Under French control from 1682–1763 and 1800–03, the area was named in honor of Louis XIV, by French explorer René-Robert Cavelier, Sieur de la Salle...

. The British

Kingdom of Great Britain

The former Kingdom of Great Britain, sometimes described as the 'United Kingdom of Great Britain', That the Two Kingdoms of Scotland and England, shall upon the 1st May next ensuing the date hereof, and forever after, be United into One Kingdom by the Name of GREAT BRITAIN. was a sovereign...

 gained authority over the region with the 1763 Treaty of Paris

Treaty of Paris (1763)

The Treaty of Paris, often called the Peace of Paris, or the Treaty of 1763, was signed on 10 February 1763, by the kingdoms of Great Britain, France and Spain, with Portugal in agreement. It ended the French and Indian War/Seven Years' War...

, marking the end of the French and Indian War

French and Indian War

The French and Indian War is the common American name for the war between Great Britain and France in North America from 1754 to 1763. In 1756, the war erupted into the world-wide conflict known as the Seven Years' War and thus came to be regarded as the North American theater of that war...

.