Indian Territory replaces Missouri Territory
The Indian Territory and the Indian Territories are terms that generally described an evolving land area set aside by the United States government for the relocation of Native Americans who held original Indian title to their land as a sovereign independent state. The tribes ceded land they occupied in exchange for land grants in 1803. The concept of an Indian Territory was an outcome of the U.S. federal government's 18th- and 19th-century policy of Indian removal. After the American Civil War (1861–1865), the policy of the U.S. government was one of assimilation.
The term Indian Reserve describes lands the British set aside for Indigenous tribes between the Appalachian Mountains and the Mississippi River in the time before the American Revolutionary War (1775–1783).
Indian Territory later came to refer to an unorganized territory whose general borders were initially set by the Nonintercourse Act of 1834, and was the successor to the remainder of the Missouri Territory after Missouri received statehood.  The borders of Indian Territory were reduced in size as various Organic Acts were passed by Congress to create incorporated territories of the United States.  The 1907 Oklahoma Enabling Act created the single state of Oklahoma by combining Oklahoma Territory and Indian Territory, ending the existence of an unorganized unincorporated independent Indian Territory as such. Before Oklahoma statehood, Indian Territory from 1890 onwards consisted of the Cherokee, Choctaw, Chickasaw, Creek and Seminole tribes and their territorial holdings.
Indian reservations remain within the boundaries of U.S. states, but largely exempt from state jurisdiction. The term Indian country is used to signify lands under the control of Native nations, including Indian reservations, trust lands on Oklahoma Tribal Statistical Area, or, more casually, to describe anywhere large numbers of Native Americans live.