I just want to make sure I understand this.
The Lakota had a treaty - specifically, the Fort Laramie Treaty of 1868 - that made their land off-limits to white settlement. The land was pursued aggressively, though (the government tried to purchase the land), after an expedition led by Custer discovered gold in the area. The Commissioner of Indian Affairs then set aside the Fort Laramie Treaty and declared that all Lakota refusing to resettle on reservations would be considered hostile.
The Lakota fought to defend their land, and eventually camped in the valley of the Little Bighorn River, where this confrontation took place.
So this was really about some people reneging on an agreement with the Indians, and getting their asses handed to them in the process.
That about right?
That sums it up about as well as it could be summed up.