The correct answer to this open question is the following.
The situation in the Black Hills was similar to the situation in Colorado at Sand Creek in that many Native American Indians were killed by white men for territorial reasons and the discovery of gold mines in both places.
In 1864, military officers led a group of 700 to kill a group of Cheyenne innocent people that were under the protection of the government. It was a massacre in which wounds cannot be forgotten by the Native Americans.
In the Black Hills, a Cheyenne Lakota territory, an expedition of George Armstrong in 1874, discovered gold in these mountains. Although the place was a Native American territory under the Laramie Treaty of 1868, white Americans and miners rushed to this region and by 1876, an armed conflict had started: the Black Hills War, better known as the Great Sioux War.