Sand Creek massacre
29 November 1865 the American’s shame, in Colorado a group of 675 men in force of cavalry, attack and destroy the Cheyenne and Arapaho village, about 70 to 165 people were killed an at least two-thirds were woman and children ( source Wikipedia), this is Sand Creek Massacre.
The Sand Creek Massacre was something horrifying and nonsense, and the result was the end of peace between native Indians and American Govern. The behavior of Colonel Chivington is a blot in the story of the United States. Of course, during the civil war, many acts have been a bud, like in all the wars, but the modality of this attack has been criminal.
Maybe can help you to understand if you read the sentence by the Congress Commission, here a small extract but you can read all in this link
“As to Colonel Chivington, your committee can hardly find fitting terms to describe his conduct. Wearing the uniform of the United States, which should be the emblem of justice and humanity; holding the important position of commander of a military district, and therefore having the honor of the government to that extent in his keeping, he deliberately planned and executed a foul and dastardly massacre which would have disgraced the veriest savage among those who were the victims of his cruelty. Having full knowledge of their friendly character, having himself been instrumental to some extent in placing them in their position of fancied security, he took advantage of their in-apprehension and defenseless condition to gratify the worst passions that ever cursed the heart of man. “
That native Cheyenne and Arapaho were pacific and they trusted of America’s promise, after the attack, The peaceful heads were replaced by young and aggressive new leaders who began a series of reprisals and heavy attacks with still human lives spent for revenge on a wicked gesture by a bad commander.