Ironically, Both of the tribes had signed a treaty with the United States just three years before in which they ceded their lands to the United States and agreed to move to the Indian reservation to the south of Sand Creek, “a line to be run due north from a point on the northern boundary of New Mexico, fifteen miles west of Purgatory River, and extending to the Sandy Fork of the Arkansas River”.
During the 1850s and 1860s, the gold rush was well underway, with many miners heading out west, angering the Cheyennes and the Arapahos. This eventually led to a war, today called the Colorado War of 1864-5. The violence between the Native Americans and the miners spread until Governor John Evans decided it was time to call out Colonel John Chivington to quiet the unruly Indians. After a few skirmishes and a decisive warpath on the part of the Indians, the Cheyennes and Arapahos were ready for peace and camped near the forts.
Black Kettle, a chief of a group of assorted Arapahoes and Cheyennes, some 600 in number, reported to Fort Lyon in an effort to declare peace. After having done so, he and his band camped out at nearby Sand Creek, less than 40 miles north. Having heard the Indians had surrendered, Chivington and his troops marched to their campsite in order to declare an easy victory for himself. On the morning of November 29, 1864, The Colorado Third Cavalry shot down people as if they were buffalo, killing as many as 200, or one-third of the original population.
See also, Indian Wars