Why the US Army tried to exterminate the bison

In 1894, a notorious poacher, Ed Howell, was caught in Yellowstone National Park slaughtering bison, which were on the brink of extinction. US Army soldiers patrolling the park brought him into custody, and the story led to the first US federal law protecting wildlife. The soldiers were thought of as heroes for stopping the killer. But it was the US Army who had been responsible for driving bison to near-extinction in the first place.