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 beginning in the mid-1840s until the late-1860s 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 planning of the exodus in 1846, the territory was part of the Republic of Mexico, with which the U.S. soon went to war over a border dispute left unresolved after the annexation of Texas. The Salt Lake Valley became American territory as a result of this war. The journey was taken by about 70,000 people beginning with advance parties sent out by church leaders in March 1846 after the 1844 death of the church's leader Joseph Smith made it clear that the group could not remain in Nauvoo, Illinoiswhich 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. Background of the migration Since its founding in 1830, members of the LDS Church frequently had conflicts and difficult relations with non-members, due to both their unorthodox religious beliefs and the conduct of the church leaders and members. 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 they built the city of Nauvoo. Sidney Rigdon was the First Counselor in the church's 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