The Mountain Meadows massacre involved a mass slaughter of the Fancher-Baker emigrant wagon train at Mountain Meadows in the Utah Territory by the local Mormon militia in September 1857. It began as an attack, quickly turned into a siege, and eventually culminated on September 11, 1857, in the execution of the unarmed emigrants after their surrender. The Arkansas emigrants were traveling to California shortly before the Utah War started. Mormons throughout the Utah Territory had been mustered to fight the United States Army, which they believed was intending to destroy them as a people.
Source: Imam Wikipedia