In 1793, Eli Whitney invented the Cotton Gin, which exponentially increased cotton production in the South to the point that much of their economy was based on it, leading to a demand for more and more slaves to harvest that cotton. This grew Southern economic dependency on slaves which bred their hardline stance that would become the basis for the sectional tensions behind the civil war.
1854: Establishment of the Republican Party. Like-minded individuals such as Anti-Slavery Whigs and Northern Democrats met to establish a new party that stood for Northern interests and Anti-Slavery ideals. They created the Republican party in Wisconsin on March 20, 1854. Many would rise to prominence in this party including future President Abraham Lincoln. This party stood for everything the Southern people disagreed with, so it's creation in and of itself was a thorn in the side of Southern interests in Government. Furthermore, its platforms frightned Southerners into action, causing them to make decisions iike that to secede upon the election of Lincoln.
In 1852, Harriet Beacher Stowe published her famous novel: Uncle Tom's Cabin. This work demonized slavery and humanized slaves in the eyes of Northerners and was seen as a blatant propaganda move by the South. Either way, it was the most successful book written on the topic of slavery at the time, making it remarkable due to the fact it was written by a white woman. Regardless, it resonated with many in the North who turned their backs on the morally corrupt South.
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In the Landmark 1857 Ruling in Dred Scott v. Sandford, the U.S. (but Southern Controlled) Supreme Court decided that black individuals were not allowed the rights of American citizens, and to a certain extent not even the rights of human beings. This ruling was appalling to Northerners, and deepened the idea that slave power controlled the government, and the only choice to fight the South was war.
In 1860, Lincoln was elected. This marked the first time a Republican had ever held that office in American history. It was hotly contested but he carried the more populous North, so the electoral college gave him the win. While he affirmed that he would protect the right to slavery in the South, the Lower South, in fear of Republican dominance, seceded from the Union, beginning the first part of the Civil War.
In 1858, James Hammond proclaimed his Mudsill Theory. He believed that there must always be a lower class in order to preserve the upper class and drive civilization forward, and while he did not specifically say that these were the non-whites of America, it was heavily implied. This idea of fixed hierarchy in life was rejected by Northern thinkers and led to a greater divide in Northern and Southern outlooks on the social system of the Union.