Geography of North and South.
The United States had become a nation of two distinct regions. The free states in New England, the Northeast, and the Midwest had a rapidly growing economy based on family farms, industry, mining, commerce and transportation, with a large and rapidl... y growing urban population. Their growth was fed by a high birth rate and large numbers of European immigrants, especially British, Irish and German. The South was dominated by a settled plantation system based on slavery. There was some rapid growth taking place in the Southwest (e.G., Texas), based on high birth rates and high migration from the Southeast, but it had a much lower immigration rate from Europe. The heavily rural South had few cities of any size, and little manufacturing except in border areas. Slave owners controlled politics and economics, although about 75% of Southern white families owned no slaves and usually were engaged in subsistence agriculture.
Overall, the Northern population was growing much more quickly than the Southern population, which made it increasingly difficult for the South to continue to influence the national government. By the time of the 1860 election, the heavily agricultural southern states as a group had fewer Electoral College votes than the rapidly industrializing northern states. Lincoln was able to win the 1860 Presidential election without even being on the ballot in ten Southern states. Southerners felt a loss of federal concern for Southern pro-slavery political demands, and their continued domination of the Federal government was threatened. This political calculus provided a very real basis for Southerners' worry about the relative political decline of their region due to the North growing much faster in terms of population and industrial output.
In the interest of maintaining unity, politicians had mostly moderated opposition to slavery, resulting in numerous compromises such as the Missouri Compromise of 1820. After the Mexican-American War of 1846 to 1847, the issue of slavery in the new territories led to the Compromise of 1850. While the compromise averted an immediate political crisis, it did not permanently resolve the issue of the Slave power (the power of slaveholders to control the national government on the slavery issue). Part of the 1850 compromise was the Fugitive Slave Law of 1850, requiring that Northerners assist Southerners in reclaiming fugitive slaves, which many Northerners found to be extremely offensive.
Amid the emergence of increasingly virulent and hostile sectional ideologies in national politics, the collapse of the old Second Party System in the 1850s hampered efforts of the politicians to reach yet one more compromise. The compromise that was reached (the 1854 Kansas-Nebraska Act) outraged many northerners, and led to the formation of the Republican Party, the first major party with no appeal in the South. The industrializing North and agrarian Midwest became committed to the economic ethos of free-labor industrial capitalism.
Arguments that slavery was undesirable for the nation had long existed, and early in U.S. history were made even by some prominent Southerners. After 1840, abolitionists denounced slavery as not only a social evil but a moral wrong. Many Northerners, especially leaders of the new Republican Party, considered slavery a great national evil and believed that a small number of Southern owners of large plantations controlled the national government with the goal of spreading that evil. Southern defenders of slavery, for their part, increasingly came to contend that blacks actually benefited from slavery, an assertion that alienated Northerners even further