From The Earth Is Weeping: The Epic Story of the Indian Wars for the American West, by Peter Cozzens (Knopf, 2016), Kindle Loc. 475-495:
The most powerful newcomers before the whites spilled onto the plains were the Sioux, formerly a woodlands people of the present-day upper Midwest. As it shifted west, the Sioux nation separated into three divisions: the Dakotas, a semisedentary people who clung to the Minnesota River; the Nakotas, who settled east of the Missouri River; and the Lakotas, who wrestled their way onto the northern plains. The Lakotas were the true horse-and-buffalo Sioux of popular imagination, and they constituted nearly half the Sioux nation. The Lakotas in turn divided into seven tribes: the Oglalas, Brulés, Miniconjous, Two Kettles, Hunkpapas, Blackfeet, and Sans Arcs, of which the Oglalas and the Brulés were the largest. In fact, these two tribes alone outnumbered all the non-Lakota Indians on the northern plains.
In their westward march across present-day Nebraska and the Dakotas during the early nineteenth century, the Lakotas gradually allied themselves with the Cheyennes and the Arapahos, who had been pushed onto the northern plains in advance of the Lakotas and had already forged an enduring bond, albeit an odd coupling. Their languages were mutually unintelligible, an impediment they overcame with a sophisticated sign language, and their characters could not have been more dissimilar. The Arapahos tended to be a kindly and accommodating people, whereas the Cheyennes evolved into fearsome warriors. The first contact between the Lakotas and the Cheyenne-Arapaho combination was hostile, because they competed for the game-rich Black Hills country. “Peace would be made,” a Cheyenne chief recounted. “They would hold out the pipe to us and say, ‘Let us be good friends,’ but time and again treacherously broke their promises.” Not until the 1840s did the Lakotas keep their word. By then, many of the Cheyennes and Arapahos, fed up with the duplicity of the Lakotas and lured by white traders, had migrated south, forming the Southern Cheyenne and Southern Arapaho tribes and leaving the Lakotas the undisputed suzerains of the northern plains.
The Lakotas and the Cheyennes and Arapahos who remained on the northern plains had the same tribal enemies—the badly outnumbered but hard-fighting Crows of present-day central Montana and northern Wyoming and the semi-agricultural Pawnees who dwelled along the Platte River in Nebraska. The basis of the rivalry was both a relentless drive by the Lakota–Northern Cheyenne–Northern Arapaho alliance to expand their hunting lands and the warrior culture common to all Plains tribes. Geographically separated from each other, the Crows and the Pawnees never formed an alliance, but being badly in need of friends—or enemies of their enemies conceived of as friends—both tribes instead eventually cast their fate with the whites.