Why did the sioux agree to live on reservation

The Great Sioux Reservation comprised all of present-day South Dakota west of the Missouri River, including the sacred Black Hills and the life-giving Missouri River. Under article 11 of the 1868 Fort Laramie Treaty, the Great Sioux Nation retained off-reservation hunting rights to a much larger area, south to the Republican and Platte Rivers, and east to the Big Horn Mountains.  Under article 12, no cession of land would be valid unless approved by three-fourths of the adult males. Nevertheless, the Congress unilaterally passed the Act of February 28, 1877 (19 stat. 254), removing the Sacred Black Hills from the Great Sioux Reservation.  The United States never obtained the consent of three-fourths of the Sioux, as required in article 12 of the 1868 Treaty. The U.S. Supreme Court concluded that "A more ripe and rank case of dishonorable dealings will never, in all probability, be found in our history."  United States v. Sioux Nation of Indians, 448 U.S. 371, 388 (1980).

Explanation:

i think it was   tsar alexander ii