Final answer:
Southern whites felt threatened by the Missouri Compromise due to fears it could lead to the eventual abolition of slavery and limit its expansion, thus upsetting the economic and political balance that the institution of slavery provided to the South.
Step-by-step explanation:
Why Southern Whites Felt Threatened by the Missouri Compromises
The Missouri Compromise of 1820 was pivotal in the growing sectional conflict between the North and the South over the issue of slavery.
This compromise allowed Missouri to enter the Union as a slave state while Maine was admitted as a free state, maintaining a balance of power in the Senate.
However, it also established the 36° 30' parallel as a boundary above which slavery was prohibited in the remaining territories of the Louisiana Purchase, excluding Missouri.
Southern whites felt threatened by the Missouri Compromise primarily because they believed it limited the expansion of slavery into new territories, potentially upsetting the delicate equilibrium between slave and free states in future states' admission to the Union.
This fear was articulated through a belief that the federal government might eventually move towards abolishing slavery, a policy that would undermine the economic foundation of southern society, based significantly on the institution of slavery, especially after the invention of the cotton gin had revitalized the South's dependence on slave labor.
Over time, the sentiment hardened with proponents of slavery advocating the idea of popular sovereignty to determine whether a territory would permit slavery, effectively seeking to extend slavery's reach.
Views like those of Thomas Jefferson, who fretted that the compromise could be 'the knell of the Union,' reflected the deepening divisions.
These resentments would eventually contribute to the breakdown of the sectional peace brokered by compromises like those of 1820 and the eventual slide towards the Civil War.