nation at-large as an unjustifiable massacre of peaceful Indians under Peace Chiefs such as Chief Black Kettle of the Southern Cheyenne Tribe. Tappan was appointed to head the military commission that investigated Colonel Chivington for his role in the action at Sand Creek. After testifying to Tappan and his fellow Commissioners at the inquiry against Chivington held in Denver, Capt. Silas Soule, a fellow Kansas pioneer and abolitionist, was killed by Charles W. Squires, another of the Colorado volunteers, presumably for his testimony against Chivington. Lieutenant Joseph Cramer and Major Wynkoop also testified against Chivington at the inquiry at which Tappan presided. The investigation produced a report that was shared with the Military command as well as the US Congress. Two separate Congressional investigations were also undertaken of the events at Sand Creek both of which excoriated Chivington's actions. Chivington was never punished as he had been mustered out of the service in January 1865 prior to the initiation of the investigative military commission. Tappan was later brevetted to the rank of Colonel before he was mustered out of the army at the end of the Civil War. During his service in the First Colorado Cavalry, elements of the regiment also joined the New Mexico Volunteers in campaigns against various Plains tribes, including the Kiowa, Comanche, and Apache. Native American rights activities Both as a military officer and a civilian, Tappan was involved in negotiations and treaties between Native Americans and the United States government. While serving at Fort Garland, Tappan commented on the enslavement of Navajos by both the Utes and Mexicans, and sought official support to try to end the practice. He acted as witness to the treaty executed on October 7, 1863, at Conejos between the Tabeguache band of Utes under Ouray and the US government represented by Colorado Territory's Governor Evans, New Mexico Territory's Indian Superintendent Dr. Michael Steck, Indian agents Simeon Whiteley and Lafayette Head, and President Lincoln's personal representative John G. Nicolay. Tappan was later appointed by President Andrew Johnson in July 1867, pursuant to an act of the US Congress, to serve as a member of the Indian Peace Commission along with Generals William Tecumseh Sherman, William S. Harney, Alfred H. Terry, and C. C. Augur and several prominent civilians. The Commission negotiated with several tribes in the Plains during 1867–1868, as well as other native tribes in the Southwest. In Kansas, on