In 1849, the Mormons who had recently settled the Wild West near the Great Salt Lake, ‘proposed’ the state of Deseret. It’s not clear to me whether this ‘proposal’ equalled a ‘proclamation’ in their minds. It is clear however that the United States never recognised the State of Deseret during the 2 year period it was administered by the Mormons – although this was the very goal for which they had wished to set up Deseret.
Deseret, by the way, is not a corruption of the word desert (which can be used to describe much of its territory), but derives from the Book of Mormon, a central text in the faith system of the Church of the Latter-Day Saints of Jesus Christ, as the Mormons officially designate themselves. In this Book, an American counterpart to the Bible, the word signifies honeybee.
The proposed state of Deseret was ambitious in its size, comprising most of the territory that the US recently had acquired in the Mexican Cession of 1848. To wit: almost all the surface of the present-day states of Utah and Nevada, large sections of California and Arizona, significant parts of Colorado, New Mexico, Wyoming, Idaho and Oregon.
The borders of the state were not, as the later, actual states, straight-lined, but to a large degree determined by natural features: the Rocky Mountains (to the east) and the Sierra Nevada (in the west), although Deseret would have touched the ocean south of the Santa Monica Mountains, to include the cities of Los Angeles and San Diego.
Two reasons are most often cited as to why Deseret never came to be: it would have created a state based on a separate religion, and it was probably too ambitious in its scope. In 1849, president Taylor proposed combining California and Deseret into a single Union state.
Meanwhile, the Provisional State of Deseret assumed more and more aspects of government, convoking a Great Assembly, which appointed judges, approved legislature (establishing taxes on liquor and outlawing gambling) and formed a militia.
In 1850, however, the US Congress created the Utah Territory, which encompassed only the northern part of Deseret. The Mormons acquiesced. The spiritual leader of the Mormons, Brigham Young, was inaugurated as the first governor of Utah Territory on February 3, 1851. On April 4, 1851 the General Assembly of Deseret dissolved itself and the Provisional State.
This was not the end of the dream of Deseret, though: in 1856, 1862 and again in 1872, the Mormons attempted to write a constitution that would establish a (Mormon) state of Deseret in Utah Territory. From 1862 to 1870, a group of Mormon elders met after each session of the territorial legislature as a kind of shadow government, re-ratifying the laws in the name of ‘Deseret’.
The dream of Deseret faded with the coming of the railroads, which brought many non-Mormon settlers out west. Eventually, the Utah Territory shrank to its present size, and was accepted into the Union as a state in 1896.
The boundaries of the provisional State of Deseret (orange) as proposed in 1849. The area of the Utah Territory as organized in 1850 is shaded in pink. Map taken from Wikipedia.