At the time the Battle of the Little Bighorn (or, the Battle of the Greasy Grass) was fought - on June 25, 1876 - hostilities were growing between the U.S. federal government and Native Americans who considered the Black Hills their sacred territory.
The Lakota (Sioux) and Cheyenne resented the government's interference in their lives - not to mention the appropriation of their territory (where gold had been discovered) - while the "white man" viewed "Indians" as "savage heathens."
We learn more about the tense situation, leading to the famous Battle of the Little Bighorn, from the National Park Service:
In 1875, the United States ordered all nomadic Lakota and Cheyenne to return to the Great Sioux Reservation, established by the Treaty of 1868, or be considered hostile. Thus confined they would not be a hindrance or threat to immigration into the region.
A lesser portion of the tribes determined they would continue to be independent of U.S. government authority and maintain their nomadic way of life, roaming freely throughout the Powder and Yellowstone River drainages.
The immediate issues leading to the conflict, which became known as the Great Sioux War, were the Black Hills Expedition of 1874 and the invasion of the Black Hills by gold miners.
The U.S. attempted to acquire the Black Hills by purchase but had been re-buffed by the Lakota. The Grant administration then unilaterally declared the Black Hills outside of the control of the Great Sioux Reservation.
Further, all Lakotas and Cheyennes dwelling away from the Great Sioux Reservation [were ordered to] return and live within the confines of the established boundary of the reservation, or be considered hostile.
General Philip Sheridan, the overall commander, devised a strategy that committed several thousand troops to find and engage the now-declared hostile tribes. Gen. Sheridan decided to order three Army expeditionary forces to converge on the Indian’s locations.
The resisting and freely roaming bands were scattered throughout the area of Southeastern Montana. Gen. Alfred Terry commanded the Eastern or Dakota column. Col. John Gibbon commanded the Western or Montana column. Marching from the South with the largest command was Gen. George Crook. LTC George A. Custer, and the 7th Cavalry he commanded, were part of Gen. Terry's force.
The Dakota and the Montana columns would join on the Yellowstone River in early June of 1876. Gen. Terry would assume command of the combined force and on June 21st, 1876, he would order LTC Custer and the 7th Cavalry to march up Rosebud Creek to locate the combining Lakota and Cheyenne bands.
The orders that LTC Custer received from Gen. Terry indicate that the 7th Cavalry will be a maneuvering strike force while Gen. Terry, with Col. Gibbon's Montana column, would ascend the Yellowstone and Big Horn Rivers and act as a blocking force to the North in support of LTC Custer's maneuvers.
Gen. Terry's orders allowed LTC Custer to engage the Indians his command was in pursuit of, if it seemed necessary.
In the early morning hours of June 25th, 1876, the large village of Lakotas and Cheyennes was observed from a high promontory in the Wolf Mountains. The village was 14 miles distant, to the West, in the valley of the Little Bighorn. From all indications the village was de-camping and scattering.
Fearful that the village would disperse and escape, LTC Custer ordered his command to advance. In the rush to engage the Indians, he divided the 7th Cavalry into four units in order to cover possible contingencies. The regiment went into battle piece meal. It became apparent that the assumptions of the early morning observations - that of a village escaping - were incorrect.
The village was largely intact and, from accounts, had been surprised by the approaching cavalry contingents. Fortunately the warrior fighting force was able to concentrate overwhelming numbers against a now divided Regiment and defeat it in detail.
Approximately 380 members of the 7th Cavalry survived the battle after Major Marcus Reno and Captain Frederick Benteen reunited and developed a strong defensive position on high ground.
LTC Custer and the 209 men in his immediate command were killed to a person because they had advanced to a position beyond the ability of the surviving parts of the Regiment to support them.
One year after the famous battle, Crazy Horse - the Oglala Lakota war leader who had split Custer's command in two - was also dead.
This clip, from "Custer at Little Big Horn" - part of the series "Battlefield Detectives" - uses forensic evidence, from the actual battlefield, to examine what may have caused Custer to so seriously misjudge the situation.
History Television provides more background about the series, Battlefield Detectives:
This series approaches the perennially interesting topic of famous battles in a fresh and exhilarating way. Focusing on the battlefield itself, each programme takes an important battle telling its story and posing a puzzling central question about the battle that recent scientific research is helping to illuminate - a contemporary journey of discovery and a compelling story from the past.