Danites

FAIR has a great description of who the Danites were.

The Danites were a brotherhood of church members that formed in Far West, Missouri in mid-1838. By this point in time, the Saints had experienced serious persecution, having been driven out of Kirtland by apostates, and driven out of Jackson County by mobs. Sidney Rigdon was publicly preaching that the Saints would not tolerate any more persecution, and that both apostates and mobs would be put on notice. The Danite organization took root within this highly charged and defensive environment.

The Danites are sometimes confused with the “Armies of Israel,” which was the official defensive organization that was tasked with defending the Saints. This is complicated by the fact that members of the Danite organization also served in the “Armies of Israel.”

They ended up being a bunch of vigilantes who used religion as an excuse for their illegal activities, not that dissimilar from modern-day terrorists who abuse Islam in a similar manner. Again from FAIR.

The Danites were led by Dr. Sampson Avard, and the group appears to have been formally formed about the time that Sidney Rigdon gave his “Salt Sermon” in Far West, in which he gave apostates an ultimatum to get out or suffer consequences. According to Avard, the original purpose of the band was to “drive from the county of Caldwell all that dissented from the Mormon church.” Once the dissenters had left the country, the Danites turned their attention to defending the Saints from mobs. Avard, however, took this purpose one step further by including retaliation against those who persecuted the Saints. Thus, the Danites began operating as a vigilante group outside the law. This, unfortunately, included stealing and plundering from those who stole and plundered from the Saints. The Danites believed that if they consecrated plundered goods to the Church, that they would be protected in battle. The group held secret meetings, with special signs used to identify themselves to one another.

It is important at this point to understand a bit of Missouri history. Caldwell County was created especially for the Mormons, similar to an Indian reservation, by the State of Missouri. Prior to its creation, there was no county in this area, just undefined land within the boundaries of Missouri. This was in reaction to the Mormons being driven out of their lands in Jackson county and how the politicians solved the "Mormon problem".