Why did the southern states secede from the U.S.?

The issues that ultimately led to the secession of the southern states had been brewing for a considerable amount of time. While most people (especially northerners) claimed the Civil War was all about ending slavery, the reasons that the south seceded actually had very little to do with that issue. In fact, the South had won a couple of major decisions that supported slavery, such as the Dred Scott decision in 1857 and the Fugitive Slave Law in 1850.

Perhaps the biggest issue that truly led to the secession was the debate over States' Rights. Going as far back as 1798, there had been an ongoing debate over whether or not the Federal government had the right to pass laws that contradicted laws already in place at the State level. While many people claim that the South was attempting to break up the Union, it was more accurately a case of them attempting to stand up for themselves and declare that the laws passed within a State had superiority. In fact, the preamble to the Confederate States Constitution starts with "We, the people of the Confederate States, each State acting in its sovereign and independent character..."

In 1828 Congress passed tariffs that benefited trade in the northern states. These tariffs ended up being detrimental to the Southern states that relied heavily on the ability to export their agricultural goods. In 1832, the state of South Carolina passed a state law that declared these tariffs to be invalid within the state of South Carolina. Persident Andrew Jackson reacted by sending a naval flotilla as well as the threat of federal troops to enforce the tariffs in South Carolina. This was one of the first major steps in breaking down the relationships between the North and the South. Over the next couple of decades, the South continued to suffer financially while the North benefitted, and eventually the Southern states decided they had put up with it long enough. This is why even now most Southerners refer to the American Civil War as the "War Between the States".

Interestingly enough, while it was the South that actually seceded, it was the northern states who had first threatened it. The New England states were opposed to the War of 1812, and at the Hartford Convention had actually discussed seceding themselves. Even then it was more about doing what was best for the individual states!

EDIT: In response to Doug T's impassioned argument that the war was about slavery.

It is a mistake to assume that Lincoln was "virulently anti-slavery". In fact, in his inaugural address he tried to reassure Southerners by declaring that he had no desire to interfere with slavery where it already existed. Furthermore, prior to his election, Lincoln took a very similar position when he ran for the US Senate. In his acceptance speech he stated "I believe this government cannot endure, permanently, half slave and half free." He then went on to argue that "slavery in the United States would eventually have to end everywhere or become legal everywhere in order for the nation to survive" (emphasis added). While Lincoln as an individual did not condone or approve of slavery, he also did not take an open stance against slavery.

In all fairness, I will concede that slavery was indeed a contributing factor as a reason for the Southern states seceding, and in fairness, I never meant to imply that it was not. While it was true that the institution of slavery had been supported by the Supreme Court, public opinion regarding slavery, particularly in the North, was turning against it. It was becoming increasingly apparent that the ability to expand slavery into new states as they joined the Union was going to be difficult, if not impossible. As a result, they would eventually find themselves isolated away in a country that was growing under the influence of a government that did not support their beliefs.

Once Lincoln was elected, the South decided that their voice was not going to be heard in American politics, so they decided to form their own government based on the principles that they felt benefited them the most. This meant they would be free from unfair tariffs, would have the right to decide on laws that were deemed appropriate for each state, and would permit them to continue to own their slaves as well as possibly expanding the institution by encouraging new states that were developed on the continent to join the Confederacy instead of the Union.

The reasons for the war is in Lincoln's first inaugural address. Decades prior to war, Northern politicians made it clear that the South was not going to be included in westward expansion. This idea put limits on southern political power. Lincoln said he had no issue with southern slavery where it existed. He just didn't want it going west. The south knew the country was moving westward and the day would come their political power would be over forever. This imbalance of political power was a major southern grievance. Lincoln also had a big problem after secession in the enforcement of the revenue laws. The south also opposed paying the lion share of the federal tariff that mostly benefited northern interest. The south was not going to stay in a union where they would never ever again be in power on one end and still pay unfair tariff rates on the other. Politics and economics was a major imputus for the war.

In general, I agree with the (excellent) answer by Doug T. that the failure to resolve the issue of slavery was the major issue leading up to secession. With the election of Lincoln and the Republicans, many of whom had used anti-slavery rhetoric, the Deep South in particular felt that it no longer could count on its pro-slavery perspective being adequately represented at the federal level. We can read this directly in the official justification for secession in the first state to secede, South Carolina:

Those [non-slaveholding] States have assumed the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery.... They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection.

For twenty-five years this agitation has been steadily increasing, until it has now secured to its aid the power of the common Government. ...[A]ll the States north of that line have united in the election of a man to the high office of President of the United States, whose opinions and purposes are hostile to slavery. He is to be entrusted with the administration of the common Government, because he has declared that that "Government cannot endure permanently half slave, half free," and that the public mind must rest in the belief that slavery is in the course of ultimate extinction. ...

On the 4th day of March next, this party will take possession of the Government. It has announced that the South shall be excluded from the common territory, that the judicial tribunals shall be made sectional, and that a war must be waged against slavery until it shall cease throughout the United States.

Obviously some of this is likely rhetorical exaggeration, given that the abolitionist cause was not yet as strong as implied here, but these are the actual words of justification given by the first state to secede.

However, I'd also like to address the issue of "states' rights" brought up in another answer. I actually agree that this issue played a strong role in secession, though not in the manner that many Southern apologists claim. It wasn't just that the Southern states decided to assert their individual rights of self-determination. They also justified this on the basis of previous actions by Northern states, specifically regarding the Fugitive Slave Clause of the Constitution, and the failure of the Northern states to adhere to it and its enacting laws from 1793 and 1850. Again, from the South Carolina declaration:

The Constitution of the United States, in its fourth Article, provides as follows: "No person held to service or labor in one State, under the laws thereof, escaping into another, shall, in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up, on claim of the party to whom such service or labor may be due."

This stipulation was so material to the compact, that without it that compact would not have been made. ...

The same article of the Constitution stipulates also for rendition by the several States of fugitives from justice from the other States.

The General Government, as the common agent, passed laws to carry into effect these stipulations of the States. For many years these laws were executed. But an increasing hostility on the part of the non-slaveholding States to the institution of slavery, has led to a disregard of their obligations, and the laws of the General Government have ceased to effect the objects of the Constitution. The States of Maine, New Hampshire, Vermont, Massachusetts, Connecticut, Rhode Island, New York, Pennsylvania, Illinois, Indiana, Michigan, Wisconsin and Iowa, have enacted laws which either nullify the Acts of Congress or render useless any attempt to execute them. In many of these States the fugitive is discharged from service or labor claimed, and in none of them has the State Government complied with the stipulation made in the Constitution. The State of New Jersey, at an early day, passed a law in conformity with her constitutional obligation; but the current of anti-slavery feeling has led her more recently to enact laws which render inoperative the remedies provided by her own law and by the laws of Congress. In the State of New York even the right of transit for a slave has been denied by her tribunals; and the States of Ohio and Iowa have refused to surrender to justice fugitives charged with murder, and with inciting servile insurrection in the State of Virginia. Thus the constituted compact has been deliberately broken and disregarded by the non-slaveholding States, and the consequence follows that South Carolina is released from her obligation.

To summarize, the Northern states had already been disregarding explicit Constitutional obligations and federal laws, justifying their nullification of federal law by saying that -- as states -- they could refuse to enforce immoral laws. Although such "states' rights" actions on behalf of Northern states were repeatedly overturned by federal laws and court cases (perhaps most recently in Ableman v. Booth (1859)), the Northern states kept ignoring the Constitution in this case, which the Southern states then used as justification for secession. The failure of enforcement for the Fugitive Slave Clause is mentioned repeatedly in many secession documents among the Southern states.

This directly answers the part of the question about why some states felt the Fugitive Slave Acts weren't enough, since often they were explicitly disregarded despite the instructions of the federal government.

In effect, South Carolina (and other states) actually cite the failure of the federal government to deny states' rights in this case as proof that all states were no longer adhering to the Constitution, thereby justifying a choice to secede.

Finally, and perhaps more important regarding the "states' rights" argument, we tend to forget that the Southern states did not secede en masse. They each held individual votes, and there were two primary "waves" of secession, with the "Deep South" South Carolina, Mississippi, Florida, Alabama, George, Louisiana, and Texas choosing to secede between December 20, 1860 and February 1, 1861. The remaining states (Virginia, Arkansas, North Carolina, and Tennessee) did not begin to secede until mid-April of 1861.

The reason why this is important in determining cause for secession is that these two "waves" happened for different reasons. The first wave effectively justified unilateral secession after the 1860 election and even before Lincoln took office. The second wave was not undertaken until after Lincoln called for an invasion of the South, an action that many (both Southerners and Northerners) initially called into question as potentially unconstitutional. (Lincoln's need to quell opposition to invasion ultimately led to his controversial suspension of habeas corpus, among other legally questionable actions.) Virginia, the first of the second wave of secession, initially voted down secession repeatedly during its deliberations, instead seeking to continue to work within the federal government to resolve its issues. It was only after the confrontation at Fort Sumter and Lincoln's subsequent call for 75,000 volunteers to invade the South that Virginia (and other border states) refused to send troops to invade other states and subsequently voted to secede.

This second wave of secession is a much stronger candidate for justifying the role of "states' rights" in the argument over cause. The border states largely viewed invasion as illegal and unconstitutional, and it was this matter (rather than slavery alone) that edged them toward secession instead of remaining within the union.

A rather intriguing (and first-hand) account of this position comes from Charles Francis Adams, the great-grandson of John Adams and grandson of John Quincy Adams. Charles Adams, a Northerner with an obvious impeccable Yankee pedigee, had not only fought in the Civil War but had even led a regiment of black Union soldiers. He later became president of the American Historical Association. It was during his tenure in 1902 that he wrote (and gave) an oration on why Robert E. Lee deserved a statue in Washington. (This whole oration is fascinating as a historical document from a Civil War veteran around 1900.)

Anyhow, Adams traces the detailed history of the events in the various waves of secession, and he points out the critical role Virginia's decision to not secede after Texas had:

The outlook was dark indeed; and, amid the fast gathering gloom, all eyes, all thoughts, turned to Virginia. She represented what were known as the Border States, her action it was felt would largely influence, and might control, theirs.... All depended on Virginia. This is now forgotten; none the less, it is history.

Adams goes on to describe the very day the news from Virginia's first vote was announced (February 4th), and when they voted to stay:

Virginia, speaking against secession, had emitted no uncertain sound. It was as if a weight had been taken off the mind of everyone. The tide seemed turned at last. For myself, I remember my feelings were too deep to find expression in words or sound. Something stuck in my throat.

Adams goes on to be very clear about the view most held about Virginia at that time:

Thus, be it always remembered, Virginia did not take its place in the secession movement because of the election of an anti-slavery president. It did not raise its hand against the national government from mere love of any peculiar institution, or a wish to protect and to perpetuate it. It refused to be precipitated into a civil convulsion; and its refusal was of vital moment. The ground of Virginia's final action was of wholly another nature, and of a nature far more creditable. Virginia, as I have said, made State Sovereignty an article, -- a cardinal article, -- of its political creed. So, logically and consistently, it took the position that, though it might be unwise for a State to secede, a State which did secede could not, and should not be coerced.

To us now this position seems worse than illogical; it is impossible. So events proved it.... So, now, the issue shifted. It became a question, not of slavery or of the wisdom, or even the expediency, of secession, but of the right of the National Government to coerce a Sovereign State. This at the time was well understood.

I think Adams perhaps goes a little far here, but he makes an important distinction. The "second wave" of secessions were made by states which had preferred to stay within the federal government and to continue dialogue on the issue, but in 1861 there still was a view of "patriotism" toward individual states. (Adams goes on to discuss how Lee's patriotism was clearly forced into a dilemma between his state and his country.) When Lincoln proposed to invade, he was threatening the rights of those states, and thus Virginia and several other states voted to leave the Union. (Virginia does not mention slavery at all in its declaration of secession other than noting the the Federal government had "perverted" its powers "to the oppression of the Southern slave-holding states"; Arkansas explicitly mentioned Lincoln's marshalling of troops to wage war against the seceded states as a cause.)

I don't want to give the wrong impression: slavery was still the overarching reason why this whole situation happened, and the whole reason Virginia was even debating secession was slavery. But the immediate actions that pushed the border states into secession were arguably over concern for states' rights under the threat of war and specifically federal invasion.

Lincoln's Election:

Lincoln was elected from Northern Votes alone. Combined with Northern monopolies in the other branches of government, this was the warning bell to the South that they no longer had any say in politics at all.

Taxes:

The North was industrial, and the South was agricultural. This meant that the South's exports earned more (though obviously this also meant that the North was more prepared to fight a war). The North didn't like earning less money than the South, so they taxed the South. Anytime a Southerner wanted to export their goods, they had a pay a large tariff.

The Southerners felt they were being treated like colonies with no real say in anything. So they did what they did last time they were in that situation, they seceded.

If you're interested in further reading, Facts the Historians Leave Out is a very good book on the subject.

I highly recommend Charles Dew's Apostles of Disunion which lets the words of the secession commisioners speak for themselves.

In late 1860 and early 1861, state-appointed commissioners traveled the length and breadth of the slave South carrying a fervent message in pursuit of a clear goal: to persuade the political leadership and the citizenry of the uncommitted slave states to join in the effort to destroy the Union and forge a new Southern nation.

The most common cause those comissioners themselves state is slavery.

the core of their argument—the reason the right of secession had to be invoked and invoked immediately—did not turn on matters of constitutional interpretation or political principle. Over and over again, the commissioners returned to the same point: that Lincoln's election signaled an unequivocal commitment on the part of the North to destroy slavery and that emancipation would plunge the South into a racial nightmare.