During the aftermath of the American Revolution (1775–1783), the Northern states (north of the Mason–Dixon line separating Maryland from Pennsylvania and Delaware) abolished slavery by 1804, although in some states older slaves were turned into indentured servants who could not be bought or sold. In the Northwest Ordinance of 1787, Congress (at that time under the Articles of Confederation) barred slavery from the Midwestern territory north of the Ohio River. When Congress organized the territories acquired through the Louisiana Purchase of 1803, there was no ban on slavery.