Fort Belknap Reservation was established in 1888 when Gros Ventres, Blackfeet, and River Crows ceded to the government 17,500,000 acres of their joint reservation that had covered all of northern Montana east of the Rocky Mountains. Home for the Gros Ventres and Assiniboines, who had shared hunting rights on the reservation, it was named for Wm. W. Belknap, secretary of war under President U.S. Grant. The Gros Ventres, a name given by early French fur trappers and French for "Big Belly," are also known as Atsina, and their own name for themselves is A'a'ninn, or "White Clay People." Always a small tribe, they lived in the Red River Valley, North Dakota, from 1100 to 1400 A.D., then moved west, splitting into two tribes around 1730; one group moved southwest and became the Arapaho, and the other moved northwest, ending up in Montana by the early 1800s, where they were close allies of the Blackfeet. Tradition credits the Assiniboine tribe as separating from the Yanktonai Sioux in the early 1600s after the wives of two leaders quarreled over a buffalo heart, leading the tribe to split; the western group became known as the Assiniboine. They call themselves the Nakota, meaning "The Peaceful Ones." When the reservation was created, part of the tribe enrolled there and the remainder at Fort Peck, about 180 miles to the east.