During the time that Utah was a territory rather than a state, the Federal government tended to send officials (judges and the like) to Utah who were not only not LDS, but were blatantly anti-mormon. Such officials failed to get what they considered to be their due respect. So, they sent back false reports of open rebellion to the President in Washington, D.C. They claimed that the Mormons were burning government buildings and destroying court records. President James Buchanan took these reports at face value and sent the Army to put down the "rebellion". The first word that the people of Utah got of this action was when Porter Rockwell brought the news of the Army's approach. He had gone to Independence, MO, to pickup mail for delivery to the territory. He got there to find that his contract had been canceled that the Army was on the the march. On his way back, Rockwell mixed in with the soldiers and found that the general sentiment was that they were going to Utah to once again drive the Mormons from their homes. When Rockwell carried this news back to Utah the church leadership decided (with the support of the Saints) that this time they would stand and fight. The Strategy on which they decided was a three-tiered defense. The first tier was to harass the army's supply trains. This slowed the advance of the army across Wyoming to the point at which they could not make it to Utah before the winter. The army was force to winter in the burned out Fort Bridger. This gave the Saints time to prepare the other two tiers of their defenses. The first of these was for those living in settlements north of Provo to move south and for those living south of St. George to move north. This concentrated the people in the area between the two cities. They brought all of their stored food with them and the structures in the abandoned settlements were all prepared for burning. (This included Salt Lake City.) If the army moved into any of these cities without permission, then they would be burned to the ground. (All of the previous time that the Saints had been drive out they had left behind homes, business, and farms that their oppressors could simply move into. This time would be different. The invaders would get nothing.) If the army continued its advance to Provo, then the serious fighting would begin. If they tried to go into the lands between Provo and St. George, then the Saints would start fighting it directly rather than just going after its supplies. Fortunately, it did not come to that. The first part of the strategy slowed the advance enough for the truth to get out. Reporters started writing stories exposing the falsity of the accusations of rebellion and telling of the hardship being face by the Saints as a result of the over reaction of the President and this on top of Buchanan's other missteps as President caused him to become very unpopular. A good friend to the Saints, Thomas L. Cain, traveled to Utah at his own expense to mediate an end to the dispute during the winter of 1857/8. The results of this was that the army was allowed to enter Utah, but they had to take up residence in a place that was something like 50 mile from any Mormon settlement, out in the western desert. (I have been out in the general area. It is quite desolate.) In getting there, they marched through a mostly abandoned Salt Lake City. The only people that they saw in the city were a few torch baring young men who were prepared to set the city on fire if the army diverted from their agreed upon route through the city. Additionally, Brigham Young agreed to step down as territorial governor and another man was appoint to that position. (I can't remember his name right now and I don't currently have access to my reference books.) This new governor called on the Saints to move back to the homes, but is was not until Brigham Young told them to do so that they actually did start moving back to the northern settlements. (They never did move back to the settlements south of St. George.) The army stayed in its fort in western Utah until Abraham Lincoln called them back east to help fight in the Civil War. This however was the end of Utah's isolation from the rest of the nation.

The primary reason for Johnston's Army was because the Secretary of War was a Southerner who wanted to move the bulk of the Army to the West, where they would not be readily available when the Civil War began. Porter Rockwell and several other Saints were sent by Brigham Young to slow Johnston down -- but were ordered not to harm a single person in doing so. The only deaths recorded by Johnston were accident during movement, not caused by the Mormons. One of the things that OP did was buy -- then burn down -- Fort Bridger, thus to deny it (and the supplies within) to the Army. Johnston, once on the move, disregarded orders from Washington until the President sent a political appointee who outranked General Johnston. As an aside, one of my Befores was a scout with Johnston's Army, so I'[m personally grateful that the Mormons had a "no kill" order -- scouts would have been the first targets.