In 1877, one year after the Sioux and Cheyenne defeated General George Custer at the Battle of the Little Bighorn, Talking Elk, a prominent war chief, rides in with Crazy Horse to surrender at Fort Robinson but turns back before getting there.  He can't live as the U. S. Government demands.  He turns, instead, to driving the gold miners out of the Black Hills who are there illegally.  The Black Hills, Paha Sapa, sacred land to the Sioux, was promised to them in the Fort Laramie Treaty of 1868 "for the absolute and undisturbed use and occupation of the Indians."  However, after General Custer entered the Black Hills illegally and discovered gold there in 1874, miners rushed in, violating the treaty.  Small mining camps grew into larger camps, like Deadwood and Custer, with merchants, saloons, and brothels, all in contradiction to the treaty.  Rather than remove the miners, the government decided to take the Black Hills from the Sioux, which it did illegally, passing the Congressional Act of February 28, 1877.
Other warriors join Talking Elk to form a war party.  Together, they raid the miners' operations, bringing in soldiers to protect the miners.  The Sioux are commanded to surrender to the reservation or be branded as "hostile" to be hunted down, captured, or killed.  It didn't matter to the government that the Black Hills belonged to the Sioux.  Gold was there.