Final answer:
Thomas Jefferson described the Missouri Compromise as a 'fire bell in the night' due to his belief that it was only a temporary alleviation of the sectional crisis over slavery, which foreshadowed deeper and more persistent divisions within the United States.
Step-by-step explanation:
Thomas Jefferson referred to the Missouri Compromise of 1820 as a "fire bell in the night" because he saw it as a stark warning of the growing divisions in the United States over the issue of slavery. In his correspondence with John Holmes, Jefferson expressed his fear that the Compromise marked the "knell of the Union" due to the moral and political line it drew across the nation. This line delineated which territories could and could not hold slaves, potentially deepening the rift between free and slave states. Jefferson was deeply concerned that the Compromise was only a temporary fix and that the continual conflict over slavery would lead to further polarization and possibly the dissolution of the Union.
While the Missouri Compromise was seen by some as a solution to the immediate crisis, with Missouri entering as a slave state and Maine as a free state, and a prohibition of slavery north of Missouri's southern boundary (the 36°30' line) in the Louisiana Purchase, Jefferson perceived it as a harbinger of the intractable sectional conflicts that would follow. His predictions held true, as debates over slavery and interpretations of the Constitution continued to cause strife, and the Compromise did indeed prove to be more of a reprieve than a resolution.