The real Hatfields, 1897. Source.
The Hatfield and McCoy feud is legendary in the United States, having become the subject of film and television drama. However, the events do have a root in a real feud, which took place across the Tug River, which forms the boundary between West Virginia and Kentucky in the latter half of the 19th century.
The origins of the feud are vague. Squirrel Huntin’ Sam McCoy, one of the feudists, later wrote his memories of the time, but could not remember why the feud started, but his descriptions of general horseplay and boisterousness in the context of a trial over the ownership of hogs come closest to being a general explanation.
Depending on how you count, the resulting feud left between 12 and 20 dead between 1882 and 1900, and as many wounded…
h/t HBD Chick, who blogs about — among other things — tribes and clans.
One of her readers left a comment linking to an article saying that the feud was greatly exacerbated by a genetic disorder that leads to “hair-trigger rage and violent outbursts.”