In 1862, almost all of this was still recognized as "Indian Territory" by treaty. This included Oklahoma, Kansas, Nebraska, South Dakota, North Dakota, Montana, and Wyoming.
After 1867, through the Fort Laramie Treaty and the Dawes Act, Native American Nations lost over a billion acres of land, transforming Indian Territory from a continental homeland into isolated islands of survival.
In Nebraska, for instance, from 1862 to 1867, Indigenous peoples, relative to their population in 1862, were almost eliminated by disease and colonization. The Ihanktonwan (Yankton Sioux), for instance, along with other First Nations, like the Pawnee, were forced out of Nebraska. I
In this process, the Yankton Sioux Tribe was forced to cede approximately 11–11.5 million acres, while retaining only about 430,000 acres, roughly 3.7–4% of their original homeland. In 1892–1894, the U.S. government further reduced the reservation by declaring about 168,000 acres “surplus” and taking them out of tribal control.
As a result, the Yankton land base was further compressed, leaving only fragments of their remaining territory under trust or allotment. The history of other First Nations in Canada and the USA is very similar.