The Frank T. Kelsey Papers contain the correspondence of Powder River landowner Frank T. Kelsey with retired general William Carey Brown. In December 1933, Brown, who had served on the plains with the frontier military himself, wrote to Kelsey regarding the placement of headstones on Kelsey's land to commemorate four soldiers killed during the Battle of Powder River. Since none of the soldiers were actually buried and the location of their remains unknown, the markers were cenotaphs created on the request of historian Walter Camp and had never been placed on the battlefield. Brown was also interested in having Kelsey deed a small tract of his land to the government to assure the location of the markers would remain in the future. In the course of his three month's correspondence with Kelsey, Brown mentioned maps and resources used to research the battle. Copies of these documents, along with some land ownership papers and photographs of the Kelsey family, complete this collection. Most of the photocopies have faded to near illegibility since 1964, but Brown's letters have been digitally scanned and manipulated to produce legible surrogates.

The Battle of Powder River, also known as the Reynolds Battle, occurred on March 17, 1876, in Montana Territory, United States, as part of the Big Horn Expedition. The attack on a Northern Cheyenne and Oglala Lakota Indian encampment by Colonel Joseph J. Reynolds initiated the Great Sioux War of 1876. Although destroying a large amount of Indian property, the attack was poorly carried out and solidified Northern Cheyenne and Lakota Sioux resistance to the U.S. attempt to force them to sell the Black Hills and live on a reservation.