As you know, it was slavery that drew the line and led to the war, but not quite in the way that you might think. It wasn't only a fine moral crusade, although fanatics like Crixus and John Brown viewed it as such and no more; the fact is that America rubbed along with slavery comfortably enough while the country was still young and growing (and getting over the shock of cutting loose from the mother country); it was only when the free North and the slave South discovered that they had quite different views about what kind of country the U.S.A. ought to be on that distant day when all the blank spaces on the map had been filled in, that the trouble started. Each saw the future in its own image; the North wanted a free society of farms and factories devoted to money and Yankee "know-how" and all the hot air in their ghastly Constitution, while the South dreamed, foolishly, of a massa paradise where they could make comfortable profits from inefficient cultivation, drinking juleps and lashing Sambo while the Yankees did what they dam' well pleased north of the 36' 30" line.
They couldn't both happen, not with Northern money and morality racing forward in tandem while the South stood still, sniffing the magnolias. Slavery was plainly going to go, sooner or later - unless the South cut adrift and set up shop on their own. There had been talk of this for years, and some Southerners had the amazing notion that left to them-selves they could expand south and west (for cotton needs land, by the millions of acres), embracing Mexico and the Dago countries in a vast slave empire where the white boss would lord it forever. But their wiser heads saw no need for this so long as the South controlled the Congress (and the Army), which they did because their states were united, while the Northerners were forever bickering amongst them-selves.
The situation was confused by a thousand and one political and social factors (but, believe me, you don't want to know about the Missouri Compromise or the "doughfaces" or the Taney ruling or the Western railroad or the Democratic split or the Know-Nothings or the Kansas-Nebraska Bill or the emergence of the Republican Party or the Little Giant or gradual emancipation, you really don't). It's worth noting, though, that there were folk in the South who wanted an end to slavery, and many in the North who didn't mind its continuing so long as peace was kept and the Union pre-served. Congressman Lincoln, for example, loathed slavery and believed it would wither away, but said that in the mean-time, if the South wanted it, let 'em have it; if slavery was the price of American unity, he was ready to pay it. Being a politician, of course, he had a fine forked tongue; on the one hand he spouted a lot of fustian about all men being equal (which he didn't believe for a moment), while on t'other he was against blacks having the vote or holding office or marrying whites, and said that if the two were to live together, whites must have the upper hand.
But over all, the anti-slavery feeling grew ever stronger in the North, which naturally made the South dig its heels in harder than ever. The Fugitive Slave law for recovering runaways was passed in '50, to the rage of the abolitionists; Uncle Tom's Cabin added fuel to the fire; and Crixus wasn't far out when he said that it only needed a spark to the powder-train to set off the explosion. I didn't pay him too much heed, though; what I've just been telling you was unknown to me then, and I figured Crixus's talk of gathering storms and trials by combat was just the kind of stuff that he, being a crazed abolitionist, wanted to believe.
Well, he was right, and I, in my excusable ignorance, was wrong; the storm was gathering in '59 - but what astonishes me today is that all the wiseacres who discuss its origins and inevitability, never give a thought to where it really began, back in 1776, with their idiotic Declaration of Independence. If they'd had the wit to stay in the Empire then, instead of getting drunk on humbug about "freedom" and letting a pack of firebrands (who had a fine eye to their own advantage) drag 'em into pointless rebellion, there would never have been an American Civil War, and that's as sure as any "if " can be. How so? Well, Britain abolished the slave trade in 1807, and slavery in 1833, and the South would have been bound to go along with that, grumbling, to be sure, but helpless against the will of Britain and her northern American colonies. It would all have happened quietly, no doubt with compensation, and there'd have been nothing for North and South to fight about. Q.E.D.