Between 1876 and 1877, the United States waged war with a portion of the Native American population known as the Sioux, in a conflict aptly named the Great Sioux War. Some may also know it as the Black Hills War, given the precedence for conflict involved the claim over the Black Hills in South Dakota and Wyoming. Despite the name, however, the Great Sioux War also included other Native American tribes such as the Cheyenne and the Arapaho as well as the various factions within the Lakota Sioux tribe itself, most predominantly the Oglala and Hunkpapa.
Despite the unity of common purpose in the Great Sioux War, many of the tribes who participated wavered between friendly allies and bitter rivals before the conflict took place. In fact, the Lakota Sioux had crossed the Missouri River after a smallpox epidemic and claimed the Black Hills from the Cheyenne in 1776, several years after the Cheyenne had introduced horses into Lakota society.
Originally hailing from the Great Lakes region, the Arapaho tribes ventured southward from present-day Minnesota and Manitoba and established a strong presence in the Midwest from Montana and Wyoming into the western regions of Oklahoma and Kansas. By 1811, they had allied themselves with the Cheyenne to expand their hunting territories, and by 1826 the alliance had come to include Lakota Sioux in an effort to repel Kiowa and Comanche forces from the south. The Arapaho managed peaceful relations with the Cheyenne and the Lakota, and they even dealt frequently with immigrant trading posts hosted by European Americans up until conflict arose with the United States. Expanding in all directions, the Arapaho eventually came to peaceful relations with the Comanche as well, the very tribe they once fought to stop from entering their lands, and even grew so close that a portion of the population assimilated with them, adopting the Comanche language and even coming to be known as their own individual Comanche tribe in the Texas Panhandle.
The Cheyenne and the Lakota have the more storied past, venturing between war allies and bitter enemies at different points in time. Hailing from similar lands in the Great Lakes region, the Cheyenne had ventured westward into present-day Minnesota and North Dakota. While crossing the Missouri River, they associated with the Mandan, Hidatsa, Arikara and eventually Lakota, the latter to whom they introduced horses. However, due to rising conflict between Lakota and another tribe known as Ojibwe, the Cheyenne were forced to continue migrating westward, eventually entering into conflict with the Kiowa as a result. The warring Lakota eventually crossed the Missouri River and overtook the Cheyenne, claiming their territory in the Black Hills. Forced to continue west and south, the Cheyenne eventually came into an alliance with the Arapaho as well as the opportunity to expand upon their own territory, territory that would encompass southern Montana through Wyoming and Colorado, into the western reaches of Nebraska and Kansas.
Despite past territorial disputes between the Lakota and the Cheyenne, however, they would find a common enemy in the United States military with the advent of the Great Sioux War in an effort to keep American prospectors and traders from trespassing into the Black Hills, considered sacred land by both the Cheyenne and Lakota, in search of the promise of gold. Still strongly allied with the Cheyenne, the Arapaho joined in their effort to defend the Black Hills as well in the conflict that would later be known as the Great Sioux War.