The herds were counted in tens of millions in the early nineteenth century, with credible estimates running to sixty million. By 1889 a survey put the wild population of the United States at 541 animals, and the free-ranging wild total at a few hundred. The collapse took roughly two decades and it was not incidental.
The hide trade drove it, after tanning methods in the 1870s made bison leather industrially useful and the railways made shipping viable. Army policy encouraged it in plain terms, on the reasoning that the Plains peoples could not be confined to reservations while the herds remained. Officers said so in writing, and the point was made in Congress. The destruction of a food supply as a means of subjugation is not a modern reading imposed on the period; it is what the period said it was doing.
What followed for the Plains peoples was famine and dependence on rations, and the loss of the material centre of a ceremonial system that had assumed the animal would always be there.
Restoration has been slow and is now substantial. The InterTribal Buffalo Council coordinates herds across dozens of tribal nations, Yellowstone holds the only continuously wild herd, and animals have been returned to reservation land in Montana, South Dakota and elsewhere. The bison was made the national mammal of the United States in 2016, a designation that sits oddly beside the record.