Vigilantes in the nineteenth-century American West formed self-appointed groups, or committees, to seize the duties of law enforcement and judicial authority in situations when citizens found law enforcement lacking or inadequate. These groups acted outside the normal structure of frontier government. Many such groups formed at different times and places in Montana. Committees targeted men suspected of threatening citizens' life and property. Vigilante groups held mock trials of the accused followed quickly by hanging or banishment. An effective vigilante group usually caused more criminals to flee than it hanged and left the community safer and more orderly.
Most mainstream historians make a distinction between the vigilantes who organized in the early 1860s and vigilante groups who were active in the 1880s after the territory had developed more extensive law-enforcement infrastructure. In addition, over the last twenty years, a group of historians known as "Revisionists" have challenged the motives of the Montana vigilantes that operated in Bannack-Virginia City between 1862 and 1863. R. E. Mather and F. E. Boswell's Hanging the Sheriff: A Biography of Henry Plummer (Salt Lake City: University of Utah, 1987) was the first of several works to explore this idea. The Revisionists key much of their argument on the innocence of Bannack's Sheriff Henry Plummer, hanged by vigilantes in January 1863. Revisionists charge that the vigilantes used their power to eliminate their own political enemies. Traditionalists (mainstream historians) charge that Revisionist history is highly subjective and ignores pertinent information and historical research practices. Among recent "traditionalist" scholarship is A Decent, Orderly Lynching: The Montana Vigilantes, by Frederick Allen (Norman: University of Oklahoma Press, 2004).