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Expelled from Missouri by Lilburn W. Bogg’s Missouri Executive Order 44, otherwise known as the Mormon Extermination Order, of October 27, 1838, the beleaguered Latter-day Saints sought refuge in the welcoming state of Illinois.
The Church of Jesus Christ of Latter-day Saints established itself on the banks of the Mississippi River in the swampy community called Commerce, Illinois. The new citizens changed the community’s name to Nauvoo, which is Hebrew for ‘to be beautiful,’ on April 21, 1840. Governor Thomas Carlin and the state legislature, including Abraham Lincoln, approved a petition for a municipal charter on December 16, 1840, to become effective February 1841.
Illinois Authorizes Nauvoo Legion’s Unique Provisions
The charter authorized a local militia, which became known as the Nauvoo Legion. The Twelfth Illinois General Assembly granted liberal authority to the Nauvoo Legion as an executive and legislative body as well as the customary judicial role and the state granted the mayor of Nauvoo authority to call out the militia to enforce city laws.
- SEC. 25. The city council may organize the inhabitants of said city subject to military duty into a body of independent military men to be called the "Nauvoo Legion," the court martial of which shall be composed of the commissioned officers of said legion, and constitute the law making department, with full powers and authority to make ordain, establish and execute all such laws and ordinances as may be considered necessary for the benefit, government, and regulation of said legion: Provided, Said court martial shall pass no law or act repugnant to, or inconsistent with the Constitution of the United States, or of this State, and, Provided, also, That the officers of the legion shall be commissioned by the Governor of the State. The said legion shall perform the same amount of military duty as is now, or may be hereafter required of the regular militia of the State, and shall be at the disposal of the mayor in executing the laws and ordinances of the city corporation, and the laws of the State, and at the disposal of the Governor for the public defense, and the execution of the laws of the State, or of the United States, and shall be entitled to proportion of the public arms, and, Provided, also, That said legion shall be exempt from all other military duty. (Laws of Illinois, 1840-1841, p. 57. The entire charter is also set forth in History of the Church of Jesus Christ of Latter-day Saints (B. H Roberts, ed., Salt Lake City, 1902-1912), IV: 239-46.)
The legislative power was implemented by the Nauvoo City Council on February 8, 1841, and signed by Mayor John C. Bennett.
Joseph Smith, elected by court-martial to lead the Legion, was appointed Lieutenant General. Moses K. Anderson, Illinois Adjutant General, recommended that Joseph Smith be appointed a lieutenant general. Secretary of State Lyman Trumbull issued the commission. Governor Thomas Carlin signed it.
Nauvoo Legion Activities Incite Surrounding Militias
- The parades and other activities of the legion — which included mock battles — attracted visitors from near and far. Indeed, the legion became so popular that many non-Mormons joined the ranks. At its peak, it is said to have numbered 5,000 men, the largest such body in Illinois. But there were problems. According to historian B. H. Roberts: [The Nauvoo Legion] excited the jealousy and envy of the rest of the militia in surrounding counties, and all the laudable efforts of the legion to become an efficient body with a view of assisting in the execution of the state and national laws, if occasion should require, were construed by their enemies to mean a preparation for rebellion…. Hence that which was to be a bulwark to the city, and a protection to the saints, was transformed by their enemies into an occasion of offense, and an excuse for distrusting them [CHC 2:59-60]. 
It’s important to see city, region, and state militias with an historical eye. They were ubiquitous and popular, and every man felt it his duty and honor to join at least one.
In June 1844, due to civil disturbances and mob violence from hostile neighbors, Mayor Joseph Smith mobilized the Nauvoo Legion and declared martial law in Nauvoo. Governor Ford ordered Smith’s arrest for trial on charges for which he had been exonerated twice. Smith and others surrendered themselves to the Carthage Greys, another Illinois militia, under promises of protection by Governor Ford. Some members of the Carthage Greys joined a mob who assassinated Joseph and his brother Hyrum Smith the next day, June 27, 1844, in Carthage Jail.
After the death of Joseph Smith, Brigham Young took command of the militia in August 1844. Instead of retaliation against continued mob violence, Brigham Young instructed most of the militia to leave the city. During the winter of 1844-1845, the State of Illinois revoked Nauvoo’s Charter, disallowing any Nauvoo Legion activity.
Nauvoo Legion In Utah
- The name Nauvoo Legion was revived in Utah and applied to the organized militia of the state of Deseret and later of Utah Territory. This legion was called upon in 1849 to subdue marauding Indians, and its members served in the so-called Walker War of 1853-1854, named after Wakara, a Ute chieftain. With the approach of the Utah expedition in 1857-1858, the Utah militia harassed and burned U.S. Army supply trains and prepared, if necessary, to prevent the entry of U.S. troops into Salt Lake City. In 1862, during the American Civil War, two units of the Nauvoo Legion protected overland mail and telegraph lines. Later, with a force of some 2,500 men, it fought against Indians in Utah's Black Hawk War (1865-1868). 
The 1887 Edmunds Tucker Act formally disbanded the Nauvoo Legion. In 1894, the non-sectarian National Guard of Utah became Utah’s state militia.
Flammer, Philip M. (1992), "Nauvoo Legion", in Ludlow, Daniel H, Encyclopedia of Mormonism, New York: Macmillan Publishing, pp. 997–999.
Gardner, Hamilton (Summer 1961), "Nauvoo Legion, 1840-1845 — A Unique Military Organization", Journal of the Illinois State Historical Society 54 (2): 181–197.