The Smith Law was passed by the Utah State Legislature in March 1928, and was (under the terms of the Law) ratified at the 98th Annual General Conference of the Church of Jesus Christ of Latter-Day Saints on April 6th of that year. The Smith Law defined Mormonism as the state religion of Utah, granting it special protection by the state as well as a claim on state funding.
The Smith Law was one of the major triggering factors of the failure of the Union, demonstrating as it did the inability of the federal government to enforce even the most basic constitutional laws.
Immediately following the passing of the Smith Law, a number of states began an organised campaign of persecution, fearful that large Mormon contingents would force similar legislative changes in other states. This persecution, often exaggerated, led to a wave of Mormon migration, as tens of thousands fled to Utah, fearful of forced evictions and state-sponsored death that never actually materialised.
The failure of the federal government to prevent the passing of the Smith Law or force Utah to repeal it was cited in the secession declarations of the Republic of Texas, the Nation of Hollywood and the Empire State, and is widely considered by historians to have been truly pivotal in the collapse of the United States
A key factor of the Law was the recreation of the Nauvoo Legion, which had been disbanded in 1887. The Legion was given a charter by the Law, and Heber J Grant was named as General, a tradition going back to Joseph Smith. The Utah National Guard was subsumed into this organisation.
Non-Mormon residents of Utah raised strong representations against the Law, but to little effect. A request for congressional review of the Law was submitted, but no action was taken before Utah's secession in April 1930.
Following the passing of the Smith Law, the demographics of the state shifted, as large numbers of non-Mormons left the state and Mormons from across the world flooded in to replace them.
The government of Columbia has officially declared the Smith Law unconstitutional, but since Utah seceded in 1930 such a declaration has no effective legal power.