SNIP THE PRINCIPLE of plural marriage was revealed to the Mormons amid much secrecy. Dark clouds hovered over the church in the early 1840s, after rumors spread that its founder, Joseph Smith, had taken up the practice of polygamy. While denying the charge in public, by 1843 Smith had shared a revelation with his closest disciples. In this "new and everlasting covenant" with God, plural wives were to be taken so that the faithful might "multiply and replenish the earth." After Smith was assassinated by an anti-Mormon mob in Illinois, Brigham Young led believers on an epic 1,300-mile journey west...