Why did Americans break their treaty with the Sioux regarding the Black Hills?

The Sioux peoples' treaty rights were constantly violated by gold prospectors, who kept crossing the reservation border. When they were attacked by our people defending their land, the United States government seized the Black Hills, in 1877 - illegally

The Sioux peoples' treaty rights were constantly violated by gold prospectors, who kept crossing the reservation border. When they were attacked by our people defending their land, the United States government seized the Black Hills, in 1877 - illegally