In 1874 the US Government broke the Second Fort Laramie Treaty by building a railroad through the Black Hills (an area promised to the Sioux forever).
Custer and the 7th Cavalry were posted by General Sheridan to protect the railroad’s construction from Indian sabotage. Simultaneously the engineers building the railroad discovered Gold and started a goldrush. Sitting Bull led the Sioux from the reservation in the Black Hills.
The 7th Cavalry attempted to round the Indians back into the reservations which sparked the Battle of Little Bighorn. Custer and his men were destroyed. The US was defeated and struck back leaving most Indians dead. The remaining Sioux had the Black Hills confiscated and the US annulled all previous treaties.