The Sand Creek Massacre took place in 1864 during the American Indian Wars, when Colonel Chivington charged against a village inhabited by native Indians (Cheyenne and Arapaho) and totally destroyed it, killing many people, mostly women and children. Chivington was under the ruling of Evans, the Governor of Colorado land at that time.
The local chief at Sand Creek had visited an American army fort looking for protection for his people, as claims on Indian lands were proliferating quickly and natives were sick of being pushed away all the time towards worse territories. He was told not to worry, as according to a treaty signed in 1851, Sand Creek's land had been legally granted to the Cheyennes.
But the next day he would find out that this was a false promise. A group of soldiers had surrounded Sand Creek. Right before the attack, the Indians had shown an American Flag as a symbol of their desire to keep peace and, trusting on the treaty, they had not prepared any defenses. Chivington ignored the peace intentions and gave orders to his men to kill and slaugther everybody they found in Sand Creek.
Analyzing the preambles, the chief of Sand Creek went to the fort seeking for protection as his people lacked the forces (only children and women were in town) and the military preparation to confront Colonel Chivington men, so he chose to negotiate instead. The Indians showed that their intentions were to avoid any conflict.
On the other hand, Chivington and Evans have gone down in history for comitting the worst massacres against natives. In this case, they violated their own agreement and attacked a town without any defenses. So they is no justification for their murders, in fact, Evans was forced to resign by President Andrew Johnson in 1865 after having proved his direct involvement with the Sand Creek Massacre.
