The Shawnee were Algonquin, who once lived in the Great Lakes region with others of this group, later migrated to the Ohio Valley.  At the time of first contact, the Shawnee were one of the dominant groups of the old Northwest Purchase area. Their range extended as far northeast as PA and as far south as the Carolinas, KY & TN.  Pushed westward by colonial expansion, they natually allied with other Algonquian, like the Sauk, during the times of Blackhawk [1767 - 1838] and Tecumseh [1768-1813]. 

The numerous Shawnee comprised five septs, each with its own chief and functioning much like a separate tribe yet bound together by common history, language and culture.  Plus, of course, the shared dangers posed by western expansion.  

In the 1700s, this was the primary cause of conflict within the Shawnee Nation as increasing pressure strengthened the resolve of some to stay and fight for their homelands and others to conclude that only a westward migration could save the Nation itself.

In 1779, the Shawnee Nation split irrevocably.  

About four thousand Shawnee, from all five septs,  moved west of the Mississippi to Cape Girardeau, MO.  This included most  of  the Thawegilas, Kispokothas and Peckuwes, and a sizeable portion of the Chalahgawthas.  In the history books, these became known as the Absentee Shawnee.

Less than three thousand, including perhaps 850 warriors, remained east of the Mississippi.  These included most of the Maykujays, a goodly number of Chalahgawthas, and some of the other three septs.  They became known as the Eastern Shawnee.

Before the Civil War, a band of the Absentee Shawnee settled in the western Creek lands between the two branches of the Canadian River.  After the war, when the Creeks were forced to cede their western lands, the Pottawatomies of Kansas were resettled in the same area.

Thus, a small tract of land between the North and South branches of the Canadian, between the Unassigned Lands on the west and and the Seminole Nation on the east, became known as the Pottawatomie and Shawnee Lands. 

By an 1872 Act of Congress, this was divided into Allotments.  At the time, the Shawnee and Pottawatomie together numbered about 900.  In general, the Shawnee Allottments were in the northern portion.

Links:

Shawnee History