This is perhaps my favorite hymn, in part for the message and in part for the history. You asked about the latter. I will explain what I understand of this event. On Thursday, June 27th, 1844, Joseph Smith was in Carthage jail with three of his colleagues, his brother Hyrum and two associates, John Taylor and Willard Richards. They were preparing to be tried for treason on Saturday. According to John Taylor, Joseph asked that he sing the hymn about midafternoon and, after singing, asked that he sing it again. With minor protest, he sang it the second time. In those days, it was sung to a different melody than that currently in the LDS hymnal. The four men were housed in the upper portion of the jail - the jailor's quarters - rather than the barred cell on the ground floor. Later that day, a mob likely comprising members from several local or regional militias charged the jail and killed Joseph and Hyrum, wounding John Taylor and stopping his watch at 5:16 pm. Aside from the guards at the jail, the prisoners were supposed to be protected by the Carthage Greys - a local militia reportedly known to be the most hostile to Smith and his colleagues of all those in the area but inexplicably charged with protecting the prisoners by Governor Ford. The murder of the church leaders did not quell the local atagonism toward the Latter-day Saints, and they were forced to leave their homes, farms, houses of worship, etc. a little less than two years later in what became an epic movement that eventually brought the main body of the group to the Salt Lake Valley.