Britain ceded ownership of the area to the United States as a result of Treaty of 1783 that ended the American Revolution. However it kept military posts in the region. On the Jay Treaty of 1794 it agreed to give up those posts. However it continued to support and arm hostile Indian tribes, who slowed American expansion. This was one of the major causes of the War of 1812. By defeating the British on Lake Erie (1813) and at New Orleans (1815), the US was able to reject British demands that the region be turned into an independent "Indian State" (one that Britain would effectively control). Instead the US made terms with the Indians and rapid settlement began.
Slavery was forbidden by the Northwest Ordinance of 1785.