Texas likes that it's big, and between 1860 and 1894, it was actually a million and a half acres larger than it is today. That's because during that interim, Texas insisted that the North Fork of the Red River was the southern boundary of Indian Territory. To stake the claim, Texas founded Greer County, named for a Lieutenant Governor, in 1860.
Situated between the North and South forks of the Red River, bounded by the 100th Meridian, and split by the Salt Fork of the Red River, Greer County was also in the heart of Kiowa and Comanche territory.
At first, the United States did not dispute Texas's claim to the land. According to the 1819 Adams Onis Treaty between the U.S. and Spain, all watersheds that emptied into the Mississippi River (i.e., the Louisiana Purchase) belonged to the United States. This meant that the main channel of the Red River (the South Fork) constituted the international boundary. However, the federal government created a new federal judicial district that included Greer County, Texas, which can be supposed to have legitimized Texas's claim. But then, ranchers and white settlers moved in.
The ranching activity that came to Greer County in the 1880s disrupted Kiowa and Comanche activities and put a strain on frontier protections. The U.S. also needed land to carve out Indian reservations. In 1886, Congress authorized a commission to settle the questions surrounding the boundary between Texas and Oklahoma. The commission didn't draw any conclusions, so the U.S. filed suit. In United States v. Texas (1896), the U.S. Supreme Court ruled that Greer County, Texas should be Greer County, Oklahoma Territory (Oklahoma Territory was founded in 1890 through the Organic Act in preparation for eventual statehood).
To protect Texas constituents, Congress set up homesteads in Greer County, O.T. This allowed the ranchers and settlers who were already living in Greer County, Texas at the time of the verdict to keep their claims for a filing fee, and they even had first dibs to new lands that became available in Greer County, O.T.
Kiowas and Comanches did not have that option. They lost all rights to the lands except for the Kiowa, Comanche, and Apache reservation in eastern Greer County, which was also homesteaded during this period.
Rand McNally, 1890.