High Quality Content by WIKIPEDIA articles! The Territory Northwest of the River Ohio, more commonly known as the Northwest Territory, was an organized incorporated territory of the United States that existed 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. The Congress of the Confederation enacted the Northwest Ordinance in 1787 to provide for the administration of the territories and set rules for admission as a state. On August 7, 1789, the new U.S. Congress affirmed the Ordinance with slight modifications under the Constitution.