The Great Utah War was a conflict between the United States of America and the Mormon settlers of Utah Territory beginning in 1857. Originally President James Buchanan sent an expedition of 2500 soldiers to suppress what he termed to be a "rebellion" in the Territory. These were meant to accompany Alfred Cumming whom Buchanan had selected to replace Mormon Church President Brigham Young as governor. They were also meant to build a post in Utah and function as a posse comitatus under the direction of the new governor. However, Buchanan never informed Young of his intentions, and the Latter-day Saints, fearful of annihilation by a federal army, blocked the Expedition's entrance into the Salt Lake Valley.
Although the Saints' militia, the Nauvoo Legion, first used non-violent tactics to slow the army's approach, Colonel Edmund Alexander continued to approach Utah with uncharacteristic speed. However, a violent snow storm and Mormon fortifications forced Alexander to retreat eastward to the burned out remains of Fort Bridger after the failed Battle of Echo Canyon in late October 1857. Alexander was relieved by Colonel Johnston in November, but the winter snows did not allow for a continued attack until the spring. In December, Thomas L. Kane approached Buchanan with an offer to mediate between the United States and the Mormons. But, with the loss of at least one hundred soldiers at the Battle of Echo Canyon, Buchanan did not think that he could now back down. He therefore ordered the army to put down the rebellion, sent reinforcements to Utah, and promised to "hang Brigham Young and all other traitors to the authority of the United States." In June 1858, Johnston led a successful attack through the now lightly defended Echo Canyon and reached the deserted streets of Salt Lake City. Young and a large number of followers had already begun a retreat northward toward the Bitterroot Valley in Oregon Territory in late March and had burned the city as they left. However, a large number of Saints were still on the trail northward, were encamped in the northern Utah settlements, or had been left behind as a rear guard. As Johnston fought his way through Echo Canyon, the army launched a second attack westward along the Oregon trail which caught a large number of Mormons in the open. Over 300 men, women, and children were killed by the army in what would become known as the Bear River Massacre.
Rather than pursue Young's company northward, army commanders decided instead to secure the Oregon and California trails and re-establish the connection between the eastern states and the west. They also worried about their supply lines with little left in Utah Territory to eat, and continued to be harassed by Mormon guerrillas who had taken to the mountains along with some of their Indian allies. Through late summer and fall of 1858, the army installed Alfred Cumming as the governor of Utah Territory, built several posts along the western trails, and prepared for a renewed campaign in the spring of 1859.
In the meantime, public and Congressional opinion had begun to turn against the war. Americans still generally disapproved of polygamy, and began to hear rumors of a Mormon orchestrated massacre at Mountian Meadows the year before. However, Republicans charged that the Democratic President was using his "Anti-Mormon Crusade" to divert attention away from the slavery issue and the economic Panic of 1857, and news of the Bear River Massacre brought out strong editorials denouncing the government's strategy as an attack upon innocent American citizens. Even some Democrats suggested that Buchanan had overstepped the bounds of executive power by pursuing a "war" against the Latter-day Saints without specific Congressional approval. Senator Sam Houston of Texas called the war "one of the most fearful calamities that has befallen this country, from its inception to the present moment." A congressional investigation also made it clear that Buchanan had done nothing to warn Young of the approaching army and its purposes, nor had Buchanan attempted any kind of investigation to determine whether Utah had truly been in a state of rebellion when he had sent the Expedition in the first place.
Under Congressional pressure, Buchanan sent a peace commission to Brigham Young in Oregon Territory in April, 1859. Arriving in June, the commission promised Young and his followers a full pardon if they would subject themselves to federal authority, assured them that the federal government would not interfere with their religion, and promised Young's company safe passage back to Utah.