The issue in The Smart Aleck's Guide to American History that generates the most mail is, without question, slavery and the Civil War. There are a LOT of people out there who absolutely insist that it never occurred to anyone that slavery had anything to do with what was going on at the time, and the real fight was about "State's Rights" and that thousand of black soldiers fought for the Confederacy.
Most of this is a myth - but to say that the war was "about" slavery is over simplifying. Let's take a look....
For the whole first half of the 19th century, there was no more divisive issue in America than slavery. And every ten years or so, the south would threaten to secede from the union to preserve it. The whole point of the Mexican American War was to add more "slave" states so that slave states would continue to be able to outvote non-slave states in congress.
By 1860, the growth of the country was robbing the South of its power. Today, no single region holds THAT much power, and for a president to be elected without winning any state in New England or the Southeast is hardly unheard of. But when Lincoln won without even being on the ballot in many Southern counties, people got rather freaked out.
If you read through the articles of secession that a few states issued, the reason they left was very clear: it was to preserve slavery. In fact, they usually mentioned slavery in the first couple of sentences. The new Republican party was thought of as an anti-slavery party (which wasn't totally accurate; it's like when conservatives calls the Democrats the pro-gay marriage party today), and with the country adding new states, we were approaching a situation where all the OTHER states could vote to outlaw slavery, and the south would just have to sit there and take it. This could have affected other issues besides slavery, but if you look at the documents and rhetoric from the time, it's hard to figure out which other issues, if any, were on their minds.
In fact, the CSA's constitution didn't provide many additional states' rights. In fact, by giving the president a line item veto and a six year term, it could be argued that they made their president MORE powerful (they did give him a single term, but it was largely an age of one term presidents). Really, the CSA constitution was just a minor revision of the USA one, with one particularly glaring change: a section that insured that congress would pass no law restricting slavery (see our article about the drafting of the CSA constitution, including a picture of a guy with a mullet.)
One the fighting began, no one really thought of it as a war to end slavery - they thought of it as a dust-up to get things back to "normal." But once it became clear that this was not going to be a three month conflict, but an actual war, ending slavery seemed like a good move for a variety of reasons. It gave the north a rallying cry, along with a new supply of soldiers in freed slaves. But another issue was that if the war DIDN'T end slavery, but just brought the south back to the union, we would just end up fighting the same war over again sooner or later.
Most of the "it wasn't slavery" brigade sends us the same false information - usually a quote from Grant saying that if he thought it was about slavery, he would have fought for the other side (which, as we state in a sidebar, was just a quote someone made up to make him look bad when he was running for president in 1872), or the common story that thousands of black soldiers fought for the confederacy - they weren't allowed to join at all until the very last weeks of the war, when they were desperate enough to offer freedom for service. None are known to have seen combat. Prior to that, the south was always fearful of a slave revolt, and arming black people was the last thing they wanted to do. They wouldn't even consider black union soldiers to be actual soldiers when negotiating the release of prisoners.
But that's not to say that every soldier in the CSA army was fighting to protect slavery, or that every soldier in the Union was out to end it. Indeed, your average soldier probably didn't care too much one way or the other. The reason they fought was that there was a war going on. From the point of view of a confederate soldier, there was an army marching into their home state, ready to burn down their home and everything around them. Very few of the soldiers actually owned any slaves, but all of them felt that they had SOMETHING to protect. When a ruling came down that men who owned enough slaves were exempt from service, soldiers sneered that this was a "rich man's war and a poor man's fight." And in the union, when the draft came around and men could get out of it by paying $300 (roughly a year's salary for many working class men at the time), soldiers THERE sneered that it was a "rich man's war and a poor man's fight."
Then again, most wars are.