Why do you think the American Civil War ultimately started?
The Civil War ultimately began because of the inability of leaders and statesmen in both the North and South to appreciate and understand the position of those on the opposite side of the debate; and ultimately their failure to compromise with each other. Although the immediate cause was the slavery debate; the differences between the two and resulting acrimony went much further, at least to the time of the Tariff of 1828 when South Carolina threatened to leave the Union.
Sharp differences of opinion had surfaced with the Tariff of 1828, often called the "Tariff of Abominations." At that time, John C. Calhoun had anonymously written a pamphlet entitled South Carolina Exposition and Protest, which purportedly explained the South's position on the tariff issue, but in fact polarized opinion on the issue. The slavery issue became even more polarizing, particularly from those in the North who were long on hyperbole and short on understanding. Harriett Beecher Stowe's Uncle Tom's Cabin depicted the evils of slavery, yet it did so in an unrealistic and polemic way. Henry Lloyd Garrison, publisher of The Liberator stated in its first edition:
I am aware, that many object to the severity of my language; but is there not cause for severity? I will be as harsh as truth, and as uncompromising as justice. On this subject, I do not wish to think, or speak, or write, with moderation. No! no! Tell a man whose house is on fire, to give a moderate alarm; tell him to moderately rescue his wife from the hand of the ravisher; tell the mother to gradually extricate her babe from the fire into which it has fallen;—but urge me not to use moderation in a cause like the present. I am in earnest—I will not equivocate—I will not excuse—I will not retreat a single inch—AND I WILL BE HEARD.
This type language only put Southern defenders of slavery on the defensive, and made compromise even less likely.
Additionally, both sides supported their position with arguments from the Bible; hence both sides were convinced of the righteousness of their own cause and the error if not complete moral turpitude of the other side's position.
The continuous hyperbole, religious conviction, and refusal to consider any middle ground meant that war was the only method by which the dispute between North and South could be settled. The actual casus belli was the seizure by the South of all Federal properties in southern states. When a Charleston garrison barricaded itself at Fort Sumter and refused to surrender, the shots to begin the war were fired.