What caused the great sioux war in 1876?

 It grew out of a dispute about rights to gold in the Black Hills of South Dakota; the gold was on Sioux reservation land and the United States wished to buy it back, but native leaders who were not party to the treaty establishing the reservation refused to negotiate. The first incident in the war was Custer's Last Stand, which galvanized opinion against the indigenous population 

Them great war of 1876 also inown as the black hills battle, was a series of battles and negotiatings whicj happened between 1876 and 1877 between the lakota sioux and the northern cheyene and the government of the united states hope it helps :)