Turf War of 1851
Disaster struck just outside of Denver in 1851. A turf squabble between local werewolves and invading vampires. The land was not yet claimed by the vampire government, so the move was unsanctioned (officially). Unofficially, it had been a move allowed by the former vampire king (who presently desiccates in a tomb somewhere). Local hunters intervened. Unfortunately, there was to be no peaceful resolution. The two factions were too far gone into their blood lust to stop. The wolves were trying to protect their land. The vampires wanted to kill them all.
With the utter savageness that the Hunters had witnessed in the battle, they had agreed to leave. To the North, other cities were being settled with shining new railroads. It was thought that it was better to leave the Denver area and its failing policies to the wilderness and let them burn themselves out.
The fight ended with additional vampires coming to stop their own (the bad ones were punished severely), and the wolves retreated back to the mountains. Since the area was sparsely populated, the bloodshed wasn't recorded by humans and there were only rumors circulating, most unbelievable.
Since then, very, very few Hunters remain.