The Black Hills were sacred to the Lakota.

The treaty was explicit about their sovereignty over the region.

None of this deterred the prospectors, who flooded into the Hills by the thousands, and none of it deterred the United States government, which decided to acquire the Black Hills rather than enforce the treaty.

When the Lakota refused to sell, the government issued an ultimatum: all Lakota and Northern Cheyenne living off their designated reservations must report to reservation agencies by January 31, 1876, or be considered hostile.

The ultimatum was impossible to comply with in the time given, particularly in the depths of a northern plains winter.

Many band leaders didn’t receive the message in time.

Some received it and refused it — arguing, correctly, that it violated their treaty rights.

Many simply had no way to move their people hundreds of miles through winter conditions in a matter of weeks.

When the deadline passed, the Army declared the non-compliant bands hostile and began planning a campaign to force them onto the reservations.

The non-reservation Lakota and Cheyenne, meanwhile, came together in an unprecedented gathering.