How did the power of the Plains Indians end by 1890?

The larges of the plains Indians, the Sioux, held lands extending from Oklahoma northward into Canada. Unlike east coast natives, they migrated between the seasons. Some called it "following the buffalo."

Immediately following the Civil War, the U.S. Government made a concerted effort to migrate westward and lay solid claim to the lands west of the Mississippi.

Technically, the U.S. Government owned all lands west of Missouri and the Mississippi. The government offered free land to any settler who would sit upon several acres of land for a year. This was land already occupied by the Sioux and some other smaller tribes. The Sioux, conversely, did not believe in land ownership, a common trait of all North American natives.

The U.S. government set up a series of small forts manned by cavalry as a show of safety for settlers.

In 1868 the U.S. Government entered into an agreement with the Sioux nation saying America would not attempt to occupy any more land than they did at that moment. The agreement was never honored by the American government as shown by Custer's movements in the Dakotas and his demise on June 26, 1876. But even though he was defeated, Crazy Horse, Red Cloud and other Sioux leaders, some of whom had traveled and spoken to Congress in Washington and had been reassured that the government's word would be kept.

These leaders realized that they would not prevail in the long run. Their ability to resist the white settlers decreased as the settlements increased.

By 1890 most had been herded into "reservations." These reservations were almost always land the U.S. government viewed as undesirable.