Answer:
The Great Sioux War of 1876 was caused because of the gold rush in the Black Hills, then a native territory.
Explanation:
The Black Hills were considered by the Sioux as sacred lands and were in turn a land claimed by the Lakota since their victory over the Cheyenne in 1776. In 1868, the Treaty of Fort Laramie, which concluded the Red Cloud's War, included the "Black Hills" in the Sioux Great Reserve, where non-Indians were excluded. While the Black Hills were often regarded as "terra incognita", rumors about the discovery of gold in these mountains were verified by the expedition of George Armstrong Custer in 1874.
At that time, US economy faced the Great Depression of 1873 and the miners embarked on a gold rush in the "Black Hills", in violation of treaties and federal law. These repeated intrusions on their territory, together with the recurrent inability of the United States Army to stop them, angered the Lakota and their allies. In response, Sitting Bull and Crazy Horse declared war on the intruders and the United States.
Many historians now believe that the administration of Ulysses S. Grant deliberately provoked this war because of this new gold rush. The objective would have been to open the "Black Hills" to help US economy to come out of the Great Depression.
