The Utah War (1857–1858), also known as the Utah Expedition, the Utah Campaign, Buchanan's Blunder, the Mormon War, or the Mormon Rebellion, was an armed confrontation between Mormon settlers in the Utah Territory and the armed forces of the US government. The confrontation lasted from May 1857 to July 1858. The conflict primarily involved Mormon settlers and federal troops, escalating from tensions over governance and autonomy within the territory. There were several casualties, predominantly non-Mormon civilians. Although the war featured no significant military battles, it included the tragic Mountain Meadows Massacre, where Mormon militia members killed about 120 unarmed settlers traveling to California.

The Church of Jesus Christ of Latter-day Saints, informally known as the LDS Church or Mormon Church, is a restorationist, nontrinitarian Christian denomination that is the largest denomination in the Latter Day Saint movement. The church is headquartered in the United States in Salt Lake City, Utah and has established congregations and built temples worldwide. According to the church, it has over 17 million members and over 99 thousand volunteer missionaries. As of 2012, the church was the fourth-largest Christian denomination in the U.S. As of 2023, the church reported over 6.8 million U.S. members.