The Sand Creek massacre (also known as the Chivington massacre, the battle of Sand Creek or the massacre of Cheyenne Indians) was a massacre of Cheyenne and Arapaho people by the U.S. Army in the American Indian Wars that occurred on November 29, 1864, when a 675-man force of the Third Colorado Cavalry under the command of U.S. Volunteers Colonel John Chivington, a Methodist pastor, attacked and annihilated a village of Cheyenne and Arapaho people in southeastern Colorado Territory, killing and mutilating an estimated 70 to over 600 Native American people. Chivington claimed 500 to 600 warriors were killed. However, most sources estimate around 150 people were killed, about two-thirds of whom were women and children. The location has been designated the Sand Creek Massacre National Historic Site and is administered by the National Park Service. The massacre is considered part of a series of events known as the Colorado Wars.

By the terms of the Fort Laramie Treaty of 1851 between the United States and seven Native American nations, including the Cheyenne and Arapaho, the United States recognized that the Cheyenne and Arapaho held a vast territory encompassing the lands between the North Platte River and the Arkansas River, and eastward from the Rocky Mountains to western Kansas. This area included present-day southeastern Wyoming, southwestern Nebraska, most of eastern Colorado, and the westernmost portions of Kansas.

In November 1858, however, the discovery of gold in the Rocky Mountains in Colorado, then part of the Kansas Territory, brought on the Pikes Peak Gold Rush. European immigrants flooded across Cheyenne and Arapaho lands. They competed for resources, and some settlers tried to stay. Colorado territorial officials pressured federal authorities to redefine the extent of Native American lands in the territory, and in the fall of 1860, A.B. Greenwood, Commissioner of Indian Affairs, arrived at Bent's New Fort, along the Arkansas River, to negotiate a new treaty.

On February 18, 1861, six chiefs of the Southern Cheyenne and four of the Arapaho signed the Treaty of Fort Wise with the United States, in which they ceded most of the lands designated to them by the Fort Laramie treaty. The Cheyenne chiefs included Black Kettle, White Antelope (Vó'kaa'e Ohvó'komaestse), Lean Bear, Little Wolf, and Tall Bear; the Arapaho chiefs included Little Raven, Storm, Shave-Head, Big Mouth, and Niwot, or Left Hand. The Cheyenne warriors denounced the chiefs who signed the treaty and even threatened them with death if they attempted to carry out the treaty's provisions.

The new reserve, less than 1/13 the size of the territory recognized in the 1851 treaty, was located in eastern Colorado, between the Arkansas River and Sand Creek. Most bands of the Cheyenne, including the Dog Soldiers, a militaristic band of Cheyenne and Lakota that had originated in the late 1830s, were angry at the chiefs who had signed the treaty. They disavowed the treaty—which never received the blessing of the Council of 44, the supreme tribal authority—and refused to abide by its constraints. They continued to live and hunt in the bison-rich lands of what's now eastern Colorado and western Kansas, and became increasingly belligerent over the tide of white migration across their lands. Tensions were high, particularly in the Smoky Hill River country of Kansas, along which whites had opened a new trail to the gold fields. Cheyenne who opposed the treaty said it had been signed by a small minority of the chiefs without the consent or approval of the rest of the tribe, that the signatories had not understood what they signed, and that they had been bribed to sign by a large distribution of gifts. Officials took the position that Indians who refused to abide by it were hostile and planning a war. The Cheyenne started committing minor offenses in 1861. These offenses went unpunished and, subsequently, became more significant. The desire for war was so strong with the Cheyenne that Agent Lorey urged Governor John Evans to treat the Cheyenne anew in 1863. As agreed, Governor Evans went out to meet with the chiefs, but they did not show up to the appointed place. The governor sent his guide, Elbridge Gerry, out to try to find the chiefs. Gerry returned two weeks later saying that a council had been held wherein the chiefs decided not to meet with Governor Evans. Bull Bear offered to meet with the governor, but his warriors would not allow it.

At the end of 1863 and in the beginning of 1864, word was received that a coalition was to be formed among the plains tribes to "drive the whites out of the country." In the spring and summer of 1864, the Sioux, Comanches, Kiowas, Cheyennes, and Arapahos were engaged in active hostilities which led to the murder of numerous civilians, the destruction of livestock and crops, supplies to the region being cut off, and the Colorado settlers in danger of starvation or murder at the hands of the plains tribes.

On April 12, men of the 1st Colorado Cavalry Regiment attacked Cheyenne men near Frémont's Orchard without any first attempt at parley or peace.

On April 13, a herdsman working for Irving, Jackmann & Company reported that Cheyennes and Arapahos had driven off 60 oxen and 12 horses and mules from their camp south of Denver. George Bent (a half-Cheyenne, half-white man who survived the ensuing Sand Creek Massacre along with other members of his family and tribe) disputes this version of the story. According to Bent, "One day in April some Cheyennes from one of these camps were out hunting and found a number of oxen straying about among the sand hills. As they did not know to whom the animals belonged they drove them to camp, intending to keep them until someone could lay claim to them.... The Indians had no use for the oxen; there were plenty of buffalo on that range that winter, and the Indians never would eat "tame meat" when they could get buffalo."