I will assume that you are referring to events surrounding the Sand Creek massacre that is described in Chapter 4 of this book.

The trick here was partly done by the military and partly by the civilian government.  As was often the case in relations between the Americans and the Indians, a treaty was signed.  In this case the treaty gave the Cheyenne all the land between Sand Creek and the Arkansas River.

Soldiers then came into that area and skirmishes occurred between them and the Cheyenne.  At that point, the military and the Cheyenne sat down to try to negotiate a peace settlement.

While the talks were still going on, some soldiers under Col. John Chivington attacked and massacred a village of Indians.

The word "trick" seems like a bit of an understatement for this, though...