June 26, 1858 The United States army entered Salt Lake City in order to restore peace and install Alfred Cumming (a non-Mormon) as governor.
Now I have always viewed the Utah war or Mormon war to be a microcosm of the pending American Civil War. Like the American Civil war the history of it is highly biased, depending on which side wrote the history. The Mormons have been very busy getting their side of the story out on the internet and for a "war" that most of us in the East today have never heard there is a lot of information. The war was generally over state rights and the phrase "secession from the union" was thrown around alot. Unlike the civil war both sides were not interested in being the first to draw blood and for the Military side of the war it was a bloodless war resulting in a correspondent of the New York Herald to write back; 'Killed, none; wounded, none; fooled, everybody,'
A Morman Politically Correct version of this story is at HistoryNet
A short summary of this incident of 1857-58 known as the Utah Expedition, the Utah War, the Mormon War, or Buchanan's Blunder War goes like this;
On July 24, 1847 Mormon Pioneers found Salt Lake City as the first city of the State of Deseret. Three years later the Utah territory was created and Brigham Young was appointed governor. Federal appointees were assigned to Utah and they had constant problems with the Mormons. They sent words back East about Mormon polygamy and theocratic tendencies and how they were even more peculiar and un-American then the Catholics. On March 4, 1857 James Buchanan takes office as President of the United States. He decides to replace Brigham Young as governor with Alfred Cumming and he sends in the Military. The Mormons panic and prepare for war which escalates the Military position. Heber C. Kimball, Morman, refers to 2,500 approaching troops by saying he had "wives enough to whip out the United States". The Mormons conduct guerrilla-style attacks on the Military. March 23, 1858, Brigham Young implements a scorched earth policy. All faithful are ordered to move south to Provo and to prepare their homes in Salt Lake City for burning. On June 26, 1858 Johnston's army marched through a deserted Salt Lake City and then went on to build Camp Floyd forty miles to the southwest, the Utah War was over. President James Buchanan proclaims a free pardon for the seditions and treasons heretofore committed by the Mormons.
An incident that is usually glossed over on Mormons websites is the Mountain Meadows Massacre. On September 11, 1857, more than 120 California-bound settlers from Arkansas and other states, including unarmed men, women and children, were killed in remote southwestern Utah by a group of local Mormon militiamen. They first claimed that the migrants were killed by Native Americans. The Mormon militia did not kill some small children who were deemed too young to relate the story. These children were taken by local Mormon families. Seventeen of the children were later reclaimed by the U.S. Army and returned to relatives in Arkansas. One version of the trial of John D. Lee, a participant, (the Trials were from 1875-76) involved in the massacre is here