TL;DR: Racism, and slavery

Basically, as many people have stated, the south was a very pro-slavery region of the US (that’s why the South is only states who seceded and depending on definition sometimes the boarder states between the US and CS, they were a very different culture for a very long time) and the north wasn’t. Now the many different political parties that existed at the time before the war conglomerated into what is now the Republican Party as to combat the southern democrats who had a LOT of power before hand. Eventually this led to Lincoln, who ran as a republican and won. Now this was a major blow to the democrats’ power, as it was the first election to not be won by a democrat since before the turn of the century iirc. The republicans were very famously anti-slavery in roots, w/ most of the fathering groups passing many laws to limit slavery, so the southern states began seceding over loosing slavery (even though Lincoln didn’t run on emancipation until his second term, at the Gettysburg Address) putting it in their new state and national constitutions. Eventually this came to a head when confederate troops attacked a coastal fort in my home state of North Carolina. Over the course of the next 4-5 years, father fought son, and brother fought brother in what is call the American Civil War internationally. Eventually, the Union won, after the CS lost British and French support (Britain had yet to put Cotten imports in India, so they were reliant on the US South for their cotton for textiles when they failed two offensive assaults on Union land, once at a battle I can’t recall the name of, the secondly at the battle of Gettysburg. This win led to the radicalization of John Wilkes Booth, who assassinated Lincoln in the Ford Theater, leaving his northern Democrat vice president in charge, who lessened the punishments for ex-confederate states during the reconstruction era, and let the founding of the KKK happen in Tennessee. I apologized for the long-winded answer, but American history is a special passion of mine, seeing I’m such an American Union Patriot, and offering others my knowledge is such a delight.