In 1874, Lieutenant George Armstrong Custer insisted that the Sioux give up their reservation land in the Black Hills of the Dakotas because

Lieutenant George Armstrong Custer insisted that the Sioux give up their reservation land in the Black Hills of the Dakotas because they had discovered gold in the land.

Explanation:

When the government of American state insisted that Sioux sell the reserved land located in the hills that were considered as black, however many of the leaders did not agree with him, and this led to serious battle among the two nations.

However, Caster died while in the battle, the main leader of the Sioux insisted that they should not sell their reservation to the Americans and all the leaders listened, and did not their sell reservation.