Originally Answered: Assess the extent to which slavery was the main cause of disunion and the Civil War.?
~Slave ownership was a right guaranteed by the US Constitution (See Article I, sections 2 and 9, Article IV, section 2 and Amendments IV, V, IX and X). Abolition could be brought about only by state law or by Constitution Amendment. Lincoln knew that, which is why he said throughout the 1860 campaign that he had no desire to end slavery where it existed and as president, he would have no authority to do so. Those sentiments he repeated in his First Inaugural Address.
No amendment for abolition was ever proposed during the antebellum years. Why? Because there was not enough support in the North to ratify such an amendment, forget the South. In fact, the Republican dominated northern majority in Congress passed the Corwin amendment in 1861. Corwin, if ratified, would have become Amendment XIII and would have prohibited any future amendment to abolish slavery. Lincoln wrote several letters supporting Corwin.
Only a fool believes the CSA states seceded over slavery. Why would they secede to protect that which the supreme law of the land guaranteed to them? Why secede rather than to stay in the Union and ratify Corwin, which would have guaranteed the "Perculiar Institution" for perpetuity? Logic and Common Sense make it abundantly clear that the CSA states did not secede to keep that which was already theirs and could not be taken away.
For the real reasons behind the secessions, one must harken back to the Treaty of Paris in 1783. Thirteen new nations were created when Great Britain granted independence, not a single nation. Those nations formed an alliance, a coalition or confederation of nations under the Articles of Confederation and then under the constitution. They created a central 'federal', not 'national' government to legislate over limited, carefully and expressly enumerated areas of common concern, but they did not surrender their independent sovereignty and agree to become subservient component parts of a larger whole.
The Founding Fathers almost to a man at one time or another acknowledge the right to secede from the alliance. Independence had come at too great a cost to squander it without recourse. The experiment of the Articles was a dismal failure. The Founders were too smart and to wise to enter into the new experiment of the Constitution without leaving themselves an out if it too proved to be a failure.
In using the term "state", think "nation-state", as did the founding fathers. In thinking "The United States", think 'independent (nation) states united for a common goal. The right of secession from a government which no longer serves the needs or the will of the governed is the core principle of the Declaration of Independence. Thomas Jefferson and James Madison (the former as sitting vice-president, the latter shortly after leaving Congress) authored the Kentucky and Virginia Resolutions in 1798, wherein they argued in favor of the right of the states to nullify federal law, and, at least impliedly, to secede. The New England states threatened to secede in 1803,1812, 1814 and 1815 over the Louisiana Purchase, the Embargo Acts, the Non-Intercourse Act and Mr. Madison's War. States in the north and south alike threatened to secede in 1820 over passage of the illegal and unconstitutional Missouri Compromise. On January 12, 1848, Abe Lincoln himself eloguently argued in favor of the right of secession on the floor of Congress. The right to secede was never questioned and in the early crises when the northern states threatened secession, it was southern statesmen who convinced them not to - not because the right didn't exist but because it was in everyone's best interest to keep the alliance alive.
For the reasons why the CSA states elected to secede, read the arguments raised by South Carolina during the 1837 secession crisis over the Tariff Act and Nullification Acts. The grievances of 1837 did not abate. Rather, they worsened until finally, in 1860/61 the southern states had had enough. By then, the South was paying 75% of federal taxes but 75% of federal revenues were being spent in the north. The north was unconstitutionally refusing to give full faith and credit to southern laws. The north was refusing to extradite convicted criminals and those accused of crimes to the south, if the crimes involved pertained to human chattel. Northern agitators were trying to foment rebellion and civil unrest in the south. Northern abolitionists were helping to deprive southerners of their property by helping slaves escape. (Yes, slavery was one of the focal points of the State's Rights issue.) Northern industrialists and capitalists were preventing industrialization or diversification of economy in the south and their lackeys in Congress were enacting legislation that made it difficult for the southern planters to trade on the international markets (and setting rock-bottom prices on southern goods in the domestic market). As the election of 1860 proved, the south had lost any meaningful voice in the federal government. Lincoln was elected without carrying a single state in the south and without being on the ballot in several. The House was dominated by a wide northern majority and in the Senate, the north held a majority which was all the larger if slaves states such as Missouri, Delaware, Maryland and Kentucky voted with the north. Since the Executive appoints federal and Supreme Court judges and justices, with the advice and consent of Congress, the judiciary was in the hands of the north as well, or would soon find itself there.
The democratically elected representatives of the southern people passed the Ordinances of Secession. The people ratified the Ordinances at the polls or in convention. Suddenly, 71 years of history was forgotten and the right to secede was said not to exist. Even by New York and Rhode Island, who (along with Virginia) included express reservations of the right to secede in their accessions to the Constitution (delegates of the other states said the reservations were unnecessary since the right was implicit in the Constitution itself - thought to be protected by Article IV and further guaranteed by Amendments IX and X, which amendments were under discussion at the Philadelphia (Constitutional) Convention but not included in the main body of the constitution itself).
The secessions were constitutional and legal, brought on by the tyranny of the northern majority against the interests of the south. Upon ratification of the Ordinances, the CSA states simply reclaimed the sovereignty, independence and autonomy that they had never surrendered, then they formed a new alliance of nations. The USA responded by invading in a war of aggression and conquest with the goal being the annexation of the sovereign nations of the CSA. The governments of the people, by the people, for the people of the CSA perished from the earth. It was a war of conquest by one coalition of nations against another, weaker, alliance of independent nations. It was NOT a civil war.