HISTORICAL NOTES:
The Nauvoo Legion had its origin as a militia unit originally organized in 1842 by the Mormons to defend the city of Nauvoo, Illinois. To curry political favor with the ambiguously-political Saints, the Illinois state legislature granted Nauvoo a liberal city charter that gave the Nauvoo Legion extraordinary independence. Led by Joseph Smith, Jr., founder of the Latter Day Saint movement and mayor of Nauvoo, the Legion quickly became the most formidable concentration of military power in the American West. In 1844, after a controversy stemming from the Nauvoo city council's suppression of a Nauvoo newspaper critical of Smith and his church's policies, Smith mobilized the Legion and declared martial law.
When Smith moved to claim Nancy Rigdon, daughter of Sidney Rigdon as his third wife a public brawl occurred and Smith and Rigdon were arrested and thrown into jail in Carthage.
As in most areas surrounding early Mormon history there has been much rewriting of the record over the years and debate regarding the actual involvement of the Nauvoo Legion in the jailbreak that followed, but the fact remains that Smith was killed and Rigdon released in the melee, but political posturing between Rigdon and Brigham Young over the matter left Rigdon excomunicated and Young ultimately leading the Mormons to Utah where the Nauvoo Legion retained their status as the military arm of the Mormons.
As a precursor of the coming Civil War, and perhaps one of the first major uses of the U.S. Military against civilian unrest, The Utah War, also known as the Utah Expedition, Buchanan's Blunder, the Mormon War, or the Mormon Rebellion broke out in May 1857 and lasted until July 1858. It was an armed dispute between Mormon settlers in Utah Territory and the United States government. While it had mainly non-Mormon civilian casualties, the "war" had no pitched battles and was ultimately resolved through negotiation. Nevertheless, according to historian William P. MacKinnon, the Utah War was America's "most extensive and expensive military undertaking during the period between the Mexican and Civil War, one that ultimately pitted nearly one-third of the US Army against what was arguably the nation's largest, most experienced militia."
The Nauvoo Legion were gainfully employed by the United States to protect western mail and telegraph lines from attack during the Civil War, but the unit was technically a Militia unit under the oversight of Brigham Young and the rosters do not appear in the National Archives Index.
The final service of the Legion was in Utah's Black Hawk War 1865-1868 when over 2,500 troops were dispatched against Indians led by Antonga Black Hawk. (Antonga Black Hawk was a Ute and has no connection to the Illinois Sauk chief Black Hawk of the 1830s.) In 1870 the Utah Territorial governor, J. Wilson Shaffer forced the Legion inactive unless he ordered otherwise. Federal troops dispatched in response to the 1870 Ghost Dance ensured Shaffer's order was enforced. The Nauvoo Legion never gathered again, and the 1887 Edmunds-Tucker Act permanently disbanded it. In 1894, in anticipation of statehood, the Utah National Guard was organized as Utah's official state militia.