The Northwest Territory in the United States was formed after the American Revolutionary War (1775-1783), and was known formally as the Territory Northwest of the River Ohio. It encompassed most of the pre-war British colonial territory north of the Ohio River of the Ohio Country, parts of Illinois Country, and parts of old French Canada (New France) below the Great Lakes. (These had been under French royal claims before 1763.) It was an organized incorporated territory of the United States spanning most or large parts of six eventual U.S. States. It existed legally from July 13, 1787, until March 1, 1803, when the southeastern portion of the territory was admitted to the Union as the state of Ohio, and the remainder was reorganized by additional legislative actions.
In the 18th century, Great Britain and France disputed for control of this region. The French had claimed it in the 17th century as part of New France. The competition between these empires resulted in the Seven Years' War in Europe. In North America the war front was known as the French and Indian War. After Britain gained control following its defeat of France in 1763, it attempted to reserve much of this territory for use by Native Americans, under the Royal Proclamation of 1763, and for a new colony, the British Province of Quebec. This roused resentment among the British colonists of the Thirteen Colonies, who were already seeking to settle west of the Appalachian Mountains.
The region was assigned to the United States in the Treaty of Paris of 1783, but sporadic westward emigrant settlements had already resumed late in the war after the Iroquois Confederacy's power was broken and the tribes scattered by the 1779 Sullivan Expedition. Soon after the Revolution ended, land-hungry migrants started moving west. A gateway trading post developed as the town of Brownsville, Pennsylvania, which was a key outfitting center west of the mountains. Other wagon roads, such as the Kittanning Path surmounting the gaps of the Allegheny in central Pennsylvania, or trails along the Mohawk River in New York, enabled a steady stream of settlers to reach the near west and the lands bordering the Mississippi. This activity stimulated the development of the eastern parts of the eventual National Road by private investors. The Cumberland–Brownsville toll road linked the water routes of the Potomac River with the Monongahela River of the Ohio/Mississippi riverine systems in the days when water travel was the only good alternative to walking and riding. Most of the territory and its successors was settled by emigrants passing through the Cumberland Narrows, or along the Mohawk Valley in New York State.
The Congress of the Confederation enacted the Northwest Ordinance in 1787 to provide for the administration of the territories and set rules for admission of jurisdictions as states. On August 7, 1789, the new U.S. Congress affirmed the Ordinance with slight modifications under the Constitution. The territory included all the land of the United States west of Pennsylvania and northwest of the Ohio River. It covered all of the modern states of Ohio, Indiana, Illinois, Michigan, and Wisconsin, as well as the northeastern part of Minnesota. The area covered more than 260,000 square miles (670,000 km2).