He was a combat veteran in the Civil War and the Indian Wars, yet he advocated peace. He was a key figure in General George Crook's column during the 1876 Sioux War that ended the life of General George Armstrong Custer. Mills was detached to make an emergency provisioning trip to Deadwood and in the process discovered the camp of Chief American Horse at Slim Buttes. Mills led the attack on that village supported later by the rest of Crook's forces. The Battle of Slim Buttes was the first U.S. Army victory after the disaster at the Little Bighorn. In a captured teepee was found a 7th Cavalry battle flag from the Little Bighorn fight. He wrote of all of that in this autobiographical volume.