From The Feud: The Hatfields and McCoys: The True Story, by Dean King (Little, Brown, 2013), Kindle pp. 49-50:
By July 1866, Congress had reduced the army to a peacetime level of just over 54,000 men. By 1876, the number had dropped by half again, to 27,000. That year, America’s centennial celebration took a blow when the news hit the week before the Fourth of July that General George Custer had suffered a devastating defeat at the hands of two thousand Lakota and Cheyenne, under Sitting Bull, in the Montana Territory. Custer had been dispatched to open the Black Hills to gold prospectors, which the Indians, whose land it now was, hotly opposed, and to make a statement that would hit newspaper front pages from coast to coast during the presidential political conventions. Instead, Custer’s Last Stand shocked the nation.
The disputed election of Ohio Republican Rutherford B. Hayes, a former Union general, to the presidency that fall resulted in a compromise with the Democrats that ended Reconstruction and the federal occupation of the South. Army forces were shifted to the West to fight Indians and police the frontier. As America rebuilt, laid rails, and expanded, the Indians would be pushed onto smaller and more marginal reservations in the West, and the blacks, now free but left to their own devices, would be oppressed and persecuted in the South. In southern Appalachia, the isolated hill people would be conned out of their land by wealthy northeastern industrial interests, which, as the railroads opened up the region to mass extraction, swooped in and snatched up coal and timber rights before the locals had any idea what they were worth. In little more than a decade, the industrialists would wrest almost complete economic and political control of the region from the people who lived there.
IT IS NOT SURPRISING THAT the Hatfield-McCoy feud found a new spark at this juncture in history, as the strictures and safeguards of the Reconstruction era suddenly vanished. What does come as a surprise is that amid the high-risk and often turbulent work of the timbering industry, with its unbridled inebriation and rowdiness of unleashed mountain men on payday, it was a rather prosaic dispute over livestock that ignited the tinderbox of the feud.