The next tribe along the Great Plains Trail is the Arapaho.
One hundred forty-seven years ago (1864) in what is now Eastern Colorado a group of Arapaho as well as Cheyenne Indians were camped along Sand Creek. One of their leaders, Black Kettle, had negotiated with the government for peaceful relations, and all there believed they were under the protection of the U.S government, and indeed, the U.S. Army.
Early in the morning, on November 29, 1864. The U.S Army invaded the camp under the command of Colonel John Chivington. What followed were countless atrocities, many of them against women and children, and the event became known as the Sand Creek Massacre.
I include here an excellent video from 500 Nations describing the massacre.
The site where the massacre occurred is now Sand Creek Massacre National Historic Site and is administered by the National Park Service. It is the intention of the Great Plains Trail to include this site along the trail for its important historical significance.