For 157 years, a Colorado governor’s order to kill Native Americans remained on the books. Not anymore.

Throughout 1864, tensions escalated between White settlers and Native Americans on the Colorado frontier. So John Evans, the territory’s second governor, made a proclamation in June of that year, telling “friendly Indians of the Plains” to report to outposts like Fort Laramie and Camp Collins for safety and protection.

Two months later, Evans issued a decidedly darker order, authorizing all citizens of the territory to “kill and destroy, as enemies of the country, wherever they may be found … hostile Indians.”

Those proclamations led to the Sand Creek Massacre in...