The best way to think about the Civil War in general terms is as the Second Act of Americas Democratic revolution. The first act was freeing itself from the control of the British empire in the 1770s and the second act was removing the massive contradiction between the rhetoric of American freedom and the reality that 20 percent of Americas population were forcibly enslaved.
There were multiple causes to the civil war.
Cause #1 was different Economic Paths in North and South. By the beginning of the 1800s, the United States is technically one country, but its really two distinct economic systems. In the North, you have the growth and evolution of a powerful industrial and financial economies (similar to the Federalist partys economic and political path) like factories and manufacturing, small independent farmers who provide food and raw materials for manufacturing, banks, merchants, etc. This is an economy that is ADVANCING, investing in and constantly coming up with new technologies, figuring out how to offer up new products with ever greater efficiency Northern industrialists want the ability to hire and fire workers depending on the needs of their business (running factories efficiently requires adjusting the size of your workforce week to week, month to month, etc.). In the South, you have plantation agriculture BIG farms powered by the labor of enslaved African-Americans (by the 1800s the most important crop in the US is cotton)
This is similar to the Anti-Federalist desires for the nation with the major contradiction being the size and scope of plantation farming. Southern planters cling to this system because they can work slaves harder, longer, and under worse conditions, and at much lower cost than legally free, white workers. Rather than ADVANCING like the North, this is an economy that is simply EXPANDING, investing in more and more slaves working more and more land in the form of traditional agriculture. By the 1820s these two competing economic models are resulting in divisions in Congress, with most Northern Industrialists, workers, and farmers opposed to the expansion of slavery and Southern plantation owners in support of slaverys expansion
Cause #2 is Abolitionism, Slave Revolts, and the Southern Backlash. While the majority of Northerners oppose the expansion of slavery on the grounds of personal economic self-interest and a general belief in free labor by the 1830s a vocal minority starts to aggressively attack slavery on moral grounds. Another related development that caused Southerners to continue aggressive expansion rhetoric was a series of slave revolts (and revolt plans) in the first half of the 1800s. At the end of the 1700s, many Southern slave-owners, including Thomas Jefferson, believed that slavery would eventually die out on its own, and that this was a good thing. Admitted discomfort with the contradiction between a democracy and slaveholding (recognized it was inhumane).
Cause #3 dealt with the Politics of Westward Expansion. Causes #1 and #2 played out against the backdrop of westward expansion of Americans into new territories and the creation of new states, which generated the question: WHICH ECONOMIC SYSTEM WILL DOMINATE THESE NEW TERRITORIES AND STATES? FREE LABOR OR SLAVERY? The first serious political crisis that erupts over this issue happens in 1820 over that status of Missouri, which is still a territory but in the process of becoming a state (territories governed more by federal government). The second major crisis over the westward expansion issue begins in the mid-1830s and concerns the annexation of Texas and war with Mexico.
When Lincoln is elected Southern Democrats feel entirely under siege (immediate threat to slaveholding republic), and secessionist fervor escalates like South Carolina officially secedes in 1860, lower South (e.g. Mississippi) soon follows, and after the attack by Southern forces on Fort Sumter (SC) in 1861, the Upper South (e.g. Virginia) follows (much less committed to the fire-eating position, especially among lower-class whites).
Issues the Emancipation Proclamation first on September 22, 1862 because it DOES NOT free all slaves only says that slaves in any states still out of the Union at the beginning of 1863 will be freed. South remains intransigent, and Thirteenth amendment passed in January 1865 by Republican Congress. War ends in April 1865, due mostly to African-American soldiers and growing disillusionment among non-slaveholding draftees in the south. Over 600K deaths! (More than Revolution, WWI, WWII, Korea, and Vietnam combined). 13th Amendment passed by Congress early 1865 and ratified shortly after Lincolns assassination.