Although Joseph Smith was no pacifist—he preached an aggressive self-defense against the Mormon Church's many enemies—prior to September 1857, the Mormons had more often been the victims of violence than its perpetrators. That all changed with the darkest incident of the Mormon War, a series of attacks known as the Mountain Meadows Massacre.

The Mormon War
The massacre came about in the context of a rising conflict between Mormon leader Brigham Young and the federal government. When the Mormons had first arrived in the Salt Lake area in 1847, it was Mexican territory, but the U.S. soon claimed the land after the Mexican-American War. The Compromise of 1850 made Utah a U.S. territory. Brigham Young was appointed its first territorial governor in 1850 and re-appointed in 1854, but conflict soon developed between his theocracy and several non-Mormon officials who had been sent to the territory by the federal government. Those officials levied accusations of intimidation and the destruction of government documents against Young. From the beginnings of his presidency, James Buchanan judged it necessary to use force to assert federal supremacy in Utah. In the spring of 1857 he declared the territory in "rebellion," and soldiers amounting to 20 percent of the entire American army began to march west that summer. For the Mormons, this approaching force raised the specter of past "extermination orders" and state-sanctioned violence against them, and Young's followers prepared for war. In August, the Mormon leader declared himself in defiance of all "Governments, but especially ours ... I will fight them and I will fight all hell."

Preparing for War
Young had a strategy for the coming war; rather than fight the federal army directly, he would attack its supply trains. As a show of his control over the territory, he would close the trails used by emigrants when they passed through Utah on their way to California. One way of achieving this was to forbid Mormons from selling any supplies to emigrants; another was to allow friendly Indian tribes whom the Mormons had held in check to harass wagon trains that passed through, specifically permitting the Indians to take the cattle that so often accompanied the wagon trains. Although it is unclear whether Young explicitly ordered specific attacks on individual groups of migrants, the evidence suggests that in August he did authorize local Indians to essentially "have at" the wagon trains. Furthermore, Young's inflammatory rhetoric may have pushed local Mormons over the precipice from hostility to overt violence.

The Fancher Party
Into this maelstrom came an emigrant group from Arkansas, the Fancher party, slowly making its way along a route some 200 miles south of Salt Lake. A popular Mormon missionary, Parley Pratt, had recently been murdered in the emigrants' home state, and as they moved through the Utah town of Cedar City, rumors spread of insulting behavior by the migrants. In the supercharged atmosphere of war, this was enough; local Mormon militia decided to attack the emigrants with the assistance of Paiute Indians. They struck at dawn on September 7, 1857, attacking the Fancher party as it was encamped in the beautiful Alpine valley of Mountain Meadows.

The Massacre
To the surprise of militia leader John Lee and the men he commanded, the Fancher party proved itself a formidable foe, circling wagons and fending off assaults for the next few days. Even more worrisome, the emigrants had seen that Mormons were involved in the assault, and this would undercut any later claims that the attack had been solely the work of Indians. Lee and his followers decided to kill anyone old enough to testify, and then they set a trap. Lee's men offered the surrounded settlers safe passage out, then when the Arkansans agreed, the Mormons slaughtered every man, woman, and child over the age of seven, some 120 in all. The militiamen then took a vow of silence, and the cover-up began. The assault would be blamed entirely on the Paiutes, and the truth would be concealed throughout the church hierarchy. Brigham Young himself would play a part.

The Aftermath
As it turned out, the Mormon War ended quickly; U.S. soldiers marched unopposed through Salt Lake City in June 1858, Young accepted a new governor for Utah, and President Buchanan pardoned the Mormons for their "rebellion." But the stain of the Mountain Meadows massacre was not so easily erased; Lee remained a fugitive until November 1874 and went on trial for murder the next year. The trial ended in a hung jury, but then Young struck a deal with the U.S. Attorney. In exchange for receiving evidence that would confirm Lee's guilt, the prosecutor agreed not to go after any other Mormons, nor seek to implicate the church hierarchy in the massacre. Lee felt betrayed, but, in Young's words, "The time has come when they will try John D. Lee and not the Mormon Church, and that is all we have ever wanted." The militia leader was convicted in 1876 and executed in March 1877 at Mountain Meadows.