There is a Wikipedia page on the Johnson County War:
The Johnson County War, also known as the War on Powder River and the Wyoming Range War, was a range conflict that took place in Johnson County, Wyoming, from 1889 to 1893.
Johnson County is a huge county, right smack dab in the center of Wyoming - it's just east of the beautiful mountain pass town of Ten Sleep. As I entered Johnson County, I got stuck in a torrential thunderstorm, the most violent thunderstorm I've seen since I moved from the Midwest to California. It was a magnificent storm, but scary.