The Church of Jesus Christ of Latter-Day Saints, commonly known as the Mormons, first appeared during the 1820s in western New York. Its leader, Joseph Smith, created a new Christian theology that was codified in the Book of Mormon - a text he claimed to have translated from ancient tablets with divine assistance. The core of Smith's theology was that the Mormons represented a restoration of true Christian teaching and practice, thereby making them the only legitimate church. Smith added to this sense of distinctiveness by arguing that the Mormon Church represented the re-creation of Israel in North America - establishing a sort of ideological ethnicity. Part of this new Israel was the allowance of polygamy, which remained one of the most controversial aspects of the religion in the nineteenth century.
The Mormons almost immediately faced strong opposition from outside of their ranks and relocated to Ohio and Missouri before settling in Nauvoo, Illinois, in 1839. The city flourished as the capital of Mormonism but the church continued to face opposition. In 1844, Smith and his brother were murdered, dividing the Mormons into two factions. The larger portion resettled in Utah under the leadership of Brigham Young while a smaller group remained in Nauvoo until 1847, when they were forced to leave and formed the Reorganized Church of Latter-Day Saints in Independence Missouri, under Joseph Smith III's leadership.
The Mormons thrived in Utah, which they called Deseret, and quickly developed a robust society. Tensions with the federal government over polygamy and other aspects of the religion resulted in the Utah War of 1857 and subsequent federal occupation of the region until 1861.
Jan Shipps, Mormonism: The Story of a New Religious Tradition (Urbana: University of Illinois Press, 1985); Leonard J. Arrington, The Mormon Experience: A History of the Latter-Day Saints (New York: Alfred A. Knopf, 1979).