[jcranmer]

The modern state of Oklahoma was effectively the dumping ground for all of the Indian nations that were forcibly relocated, predominantly the "Five Civilized Tribes" of the Southeast who were forced out by Andrew Jackson. From 1889 to 1895, the unassigned territories, and later "unused" lands previously allotted to the relocated Indians, were given out to settlers in a series of land runs, and the involved lands were collected into an Oklahoma Territory.

In response to these pressures, the Indian Territory then attempted to turn their territory into a full state (Sequoyah), but this was turned down by Congress, which wanted the Oklahoma and Indian Territories merged into a single state instead. I believe this was motivated by political concerns, but this isn't an area of history I know very well.

It should also be noted that many of the actions the US government took during this time with respect to the Native Americans were flagrantly in violation of laws and treaties that existed, too. Any treaty the US government made with the natives were not worth the paper they were written on.