The fundamental cause of the US Civil War was the US's inability to solve slavery.

The Civil War is not fundamentally about "states rights". Asserting a state's right to secede doesn't speak to why the state wants to secede. Steven's citation of reasons only serve to underline this. When the northern states were threatened by the War of 1812, they considered secession. When South Carolina was threatened by a tarriff, they attempted to nullify the law. When a state's self interests come into play, they'll take advantage of whatever political mechanism they can imagine to assert that self interest, up to and including nullification, seccession, and war.

**The real story is not in the political mechanics but the underlying interest in preserving slavery that forced the South to become so hell bent on their "states rights".** If Northern states had seceded over the War of 1812, we wouldn't assert the fundamental cause was a debate over state's rights. Rather we'd say it was their opposition to the War of 1812. The same applies for the South's secession as well. Their interest in preserving slavery drove them to use untested constitutional mechanism and eventually go to war.

And when you consider the South's conundrum it becomes clearer.

White southerners lived in constant, real fear of slavery/black insurrection. They had experienced violence from slaves during [Turner's Rebellion][1]. They looked south to the [Haitian Revolution][2] and similar rebellions in the sugar colonies and saw little comfort in how those societies were transformed. Maintaining the institutions associated with slavery was a matter of life/death for white southerners. [Laws were passed][3] to further and further restrict the activities of slaves and freed blacks. Even if the injustice of the system might be acknowledged by some southerners, the fear that slaves or freed blacks might (perhaps justifiably seek) vengeance was deep. 

Even if a southerner thought that ending slavery was the right thing to do (and many did), the way to get there without providing massive disruption to southern society (economic or otherwise) was very hard to see. Aside from what they would have perceived as lost property, you have the question of [what do you do with the slaves][4]? If they're "free" won't they just take up arms and overthrow southern society? Would the freed blacks compete with poor whites and drag down wages? In every sense, security, economics, social interactions, slavery was at the heart of Southern life and it was apocalyptic to imagine life without it.

This need for security and tight adherence to the institution existed in direct contrast to Northern abolitionist's deep, often religiously founded, moral opposition to slavery from people such as [John Brown][5] and [others][6]. Many movements sprung up to curtail the expansion of slavery such as the [Free Soilers][7] and [Republicans][8]. The sectionalist politics of the day made southerners understand that the only way to maintain their power in Congress (and ultimately their sense of security) was to fight to keep expanding slavery to keep growing their representation.

Both the sectional conflicts around expanding slavery and the issue of slavery were ultimately irreconcilable. While campaigning Lincoln [directly attacked][9] the South's hold on Federal power through the continued expansion of slavery and the 3/5ths compromise. After Lincoln was elected, there was already a shooting war in [Kansas][10] and [John Brown][11] had raided Harpers Ferry in a vain attempt to incite a massive slave revolt. 

The South's siege mentality could only reach a fever pitch at this point. With a virulently anti slavery, pro Northern president and ever increasing and violent political conflict with people fundamentally opposed to slavery, the south felt deeply threatened politically, economically, and ultimately physically. They tried whatever mechanism they had, citing state's rights and eventually declaring war. Without doing so, they feared, they would have change forced on them through slave rebellion or Federal law. The changes felt so dangerous that they were willing to cite whatever constitutional mechanism was required and they were ultimately willing to go to war.