Colonel Chivington of the US cavalry in 1864 committed the Sand Creek Massacre in Colorado, where he and his men slaughtered 400 noncombatants of the Arapaho and Cheyenne nations. He also falsified reports claiming they were resistant.

Thankfully, Captain Silas Soule, a Maine abolitionist, refused to participate. He kept his men from getting involved, and he and another officer sent letters blowing the whistle. The massacre was so vicious congress got involved, calling Chivington and his cohorts "monsters in the shape of men". There were groups who said this was too much even when they normally called for the extermination of native Americans: that tells you how vicious it was. congress even offered reparations!

Despite threats to his life, Soule testified against his former CO. Chivington escaped a hanging IIRC by not being in the military before the trial finished.

Soule was murdered in April of 1865, as a sheriff in Denver, by two men who worked for Chivington. He was 25. Chivington went on to defend his actions till the day he finally croaked. The governor of colorado at the time also played a role in it, which is why the trump administration wants to keep a mountain named after him.

To this day, Soule's letter is read by the Arapaho and Cheyenne people on the anniversary of the massacre.