<Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (February 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> The Mormon pioneers were members of The Church of Jesus Christ of Latter - day Saints (LDS Church), also known as Latter - day Saints, who migrated across the United States from the Midwest to the Salt Lake Valley in what is today the U.S. state of Utah . At the time of the ceasefire and planning of the exodus in 1846, the territory was owned by the Republic of Mexico, which soon after went to war with the United States over the annexation of Texas . Salt Lake Valley became American territory as a result of this war . </P> <P> The journey was taken by about 70,000 people beginning with advance parties sent out by church leaders in March 1846 after the 1844 assassination of the Mormon leader Joseph Smith made it clear the group could not remain in Nauvoo, Illinois--which the church had recently purchased, improved, renamed, and developed because of the Missouri Mormon War, setting off the Illinois Mormon War . The well - organized wagon train migration began in earnest in April 1847, and the period (including the flight from Missouri in 1838 to Nauvoo), known as the Mormon Exodus is, by convention among social scientists, traditionally assumed to have ended with the completion of the First Transcontinental Railroad in 1869 . Not everyone could afford to transport a family by railroad, and the transcontinental railroad network only serviced limited main routes, so wagon train migrations to the far west continued sporadically until the 20th century . </P> <P> Since its founding in 1830, members of the LDS Church were often harshly treated by their neighbors, partially due to their religious beliefs, sometimes as a reaction against the actions and the words of the LDS Church and its members and leaders . These and other reasons caused the body of the Church to move from one place to another--to Ohio, Missouri, and then to Illinois, where church members built the city of Nauvoo . Sidney Rigdon was the First Counselor in the LDS First Presidency, and as its spokesman, Rigdon preached several controversial sermons in Missouri, including the Salt Sermon and the July 4th Oration . These speeches have sometimes been seen as contributing to the conflict known as the 1838 Mormon War in Missouri . As a result of the conflict, the Mormons were expelled from the state by Governor Boggs, and Rigdon and Smith were arrested and imprisoned in Liberty Jail . Rigdon was released on a writ of habeas corpus and made his way to Illinois, where he joined the main body of Mormon refugees in 1839 . In 1844 Joseph Smith, and his brother Hyrum Smith were killed by a mob while in custody in the city of Carthage, Illinois . In 1846, religious tensions reached their peak, and in 1848 mobs burned the Latter - day Saint temple in Nauvoo . </P>

When did the mormon pioneers cross the plains