For a good treatise on why the war happened I would suggest Tocquevilles 1832 'Democracy in America". Obviously this predates the war, but it becomes obvious rapidly why it was brewing.
The war started when the south saw that they were not going to maintain a 'half slave nation' by the introduction of new slave free states, the growing population and economy of the north, and the threat of dropping tarriffs on foreign cotton.
If you dont want to suffer through the book mentioned above (which I would suggest anyone attempt, because it is incredible), there is a very well written peice on Calhoun found here
http://militaryhistory.about.com/library/prm/blhestartedthecivilwar1.htmWhich really does a nice job. Here is a snipit from page 2.
By Ethan S. Rafuse for Civil War Times Illustrated Magazine
Calhoun hoped to use his accomplishments as war secretary as a springboard to the presidency. When that dream fell through, however, Calhoun had no problem accepting the vice presidency under staunch federalist John Quincy Adams in 1824. Adams was glad to have Calhoun in his administration, having held him in high esteem since their days together in Monroe’s cabinet. Adams was particularly impressed by Calhoun’s "ardent patriotism," believing Calhoun was "above all sectional and factious prejudices more than any other statesman of the Union with whom I have ever acted." This was an image Calhoun cultivated during the 1824 election campaign.
It turned out that Calhoun was late in publicly promoting his commitment to federalism. By this time, Southerners were increasingly taking an anti-federal-government stance. In the North, industry and the economy it created grew in influence and power every day. Meanwhile, the rapidly expanding cultivation of cotton and other cash crops was committing the South to an agrarian economy and culture, which depended on slavery. The country was dividing into two increasingly self-conscious sections with different priorities. And as the issue of slavery came to the fore in American politics, the South found itself on the defensive. Because of the South’s investment in large-scale agriculture, any attack on slavery was an attack on the Southern economy itself.
The issue came to a head in 1819 with the debate over whether to allow the Missouri Territory to become a state. The result was the historic Missouri Compromise of 1820, which permitted the territory to enter the Union as a slave state while Maine entered as a free state, maintaining the balance between free and slave states at 12 each. The compromise also prohibited slavery in the remainder of the Louisiana Purchase north of Missouri’s southern border.
On the surface, the Missouri Compromise seemed to heal the sectional breach that slavery had created. But the fact that the debate had divided along sectional lines awakened the South to the reality that it was a distinct section—a section that was apparently inevitably destined to be a minority in the Union, while the Northern states enjoyed increasing political representation and power born of rapid population growth.
In the 1820s, Southerners grew increasingly anxious about the North controlling the federal government and about how that situation threatened the South and its distinctive institutions. They looked to leaders who would limit federal power. Cal-houn unexpectedly found himself the target of sharp criticism from leading South Carolina figures, including Thomas Cooper, the president of the state college. In 1824, Cooper published a widely circulated pamphlet attacking Calhoun. "He spends the money of the South to buy up influence in the North," Cooper grumbled.
If Calhoun wanted to maintain his status as a Southern leader and reach his political goals, he could not ignore the changing political landscape. He recognized it would be a mistake to maintain his association with Adams, whose ideas to expand the use of federal power to promote national economic, intellectual, and cultural development drew a cold reception in South Carolina. So when Andrew Jackson began preparing to challenge Adams in the 1828 presidential election, Calhoun switched sides. The Democrats rewarded Calhoun by making him their candidate for vice president, and the ticket won.
That same year, Congress passed a highly protective tariff that Southerners bitterly opposed, viewing the measure as sacrificing Southern agrarian interests to benefit Northern industry. The protest against the so-called Tariff of Abominations grew particularly strong in South Carolina, and in response to a request from the state legislature, Calhoun secretly wrote an essay titled "South Carolina Exposition and Protest." In it, he asserted that states had a constitutional right to nullify any federal government actions they considered unconstitutional. Calhoun had become the chosen mouthpiece for Southern rights. Confirmation of his new status came when Congress adopted another high tariff in 1832 and South Carolina legislators used the principles Calhoun had voiced in his "Exposition and Protest" to declare the tariff "null and void."
