Why did the U. S. Take the land back that had been promised to the Native Americans in the Black Hills region of North and South Dakota?

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The U.S. took the land back that had been promised to the Native Americans in the Black Hills region of North and South Dakota because "gold was discovered and the U.S. wanted it."

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.

"the attempts of the proletariat to rise above this abasement"