Okay, ten-minute summary. In the early United States, African slavery was legal in the southern states and illegal in the northern ones. In the very beginning, there are hints that some of the southern leaders were morally embarrassed by writing things like, "all men are created equal" while simultaneously owning other people as property, but too many powerful people had too much of their money tied up in slaves (and businesses that relied on them) for emancipation to be a political possibility. The only real move against slavery that happened in the early republic was that when the Northwest Ordinance was passed, which organized the settlement of the Ohio valley and Great Lakes region, slavery was outlawed in the new territories, which later became states. (Geography note: in this era, virtually all the settlement activity was happening between the Appalachian Mountains and the Mississippi River, so when I talk about "the west," you should think of what's now the middle/east quarter of the country; what's now the western half wasn't yet the target of much settlement.)

In the next generation, though, southern leaders (who had by now become even more dependent on their slaves than their predecessors) looked at the political future and realized that if slavery was outlawed in all the new states, in a generation or two, a majority of voters would have grown up in circumstances that left them unsympathetic to slaveholders, and there would be nothing stop them from voting to abolish slavery outright, wrecking the southern economy. So when new states states were organized in the west, they tried to make sure that slavery was legal there, so that future generations in those states would be in the pro-slavery voting bloc, too.

In the north, political opinion was distinctly, but not overwhelmingly, anti-slavery, so this southern gambit didn't go off without a hitch. After a some angry political fights, a compromise was struck, where states would be admitted to the union in pairs, one slave, the other free, to preserve a balance in Congress.

This kept up for about sixty years, but during that time, the two factions became more and more hostile to one another. In the North, people grew disgusted with slavery, especially a law that forced them to hand over escaped slaves; in the South, people were indignant about northern attitudes that seemed bent on destroying their economy. At this point, some people like to point out that there were disagreements between north and south other than slavery, like tariffs and monetary policy. This is true enough, but it's worth pointing out that these disagreements really came down to whether the country should have economic policies that made sense if the country were growing a lot of cotton and indigo (which used slave labor), or that favored an industrial and financial economy (which didn't).

Anyway, by the 1850s, the two sides resented each other terribly, and what was worse, the earlier compromises about one free, one slave, state were foundering. Basically, there weren't enough slave-owning candidates for statehood, and the pipeline for new free states was bogged down waiting for them. In one prospective state, Kansas, a political fight over whether to change the laws to allow slavery devolved in a miniature civil war, with towns burnt and death squads.

Anyway, in the election of 1860, Abraham Lincoln, a northerner who favored admitting several free states to the union, was elected President. There was some electoral hijinks; not fraud, really, but the vote was split three ways, and Lincoln didn't have a true majority. And importantly, for the first time, the president had been elected without relying on the support of any of the slave states.

The leaders of the southern states decided the jig was up; Lincoln didn't suggest he was about to ban slavery right then and there, but if he got his way, and it looked like he would, there would be no more slave states. And from then, it would only be a matter of time. So they revolted– declared their independence, sent their local militias to seize all the military supplies they could, and got ready for a war. Not all the slave states went along with this; in fact, all the border states stayed in the Union (part of Virginia split off during the crisis, and became a new state, West Virginia).

At this point, I could go into more detail about the course of the war, but the short version is: the Union (northern) armies tried for a quick victory, but were defeated near Washington, DC; then the fighting went back and forth across the border, with the Confederate (southern) army pushing into Pennsylvania before being turned back. After costly setbacks, Lincoln took the gloves off; he issued the Emancipation Proclamation, which freed slaves everywhere still in revolt, and approved plans for much more aggressive tactics. Union armies under Grant and Sherman cut their way through the South's western defenses, captured several major cities (totally destroying some of them), and moved in for the kill around Richmond, Virginia. After losing a battle to defend that city, Confederate general Robert E Lee surrendered at a place called Appomattox, and the war was essentially over. Shortly after the surrender, a confederate-sympathizing actor murdered Lincoln while he was watching a b-grade play, but (shockingly!) this didn't magically reinvigorate the rebels, who had just had been comprehensively defeated.