What is the most important long term cause of the civil war?

The growing-apart of America into two very different societies, North and South.

The South entirely agricultural, growing rich on the cotton trade which was run on slave-labour, and dependent on European imports.

The North puritan in spirit, prepared to tolerate slavery in its traditional heartlands because of the cotton revenues, but hostile to the idea of an extended slave-empire - partly for ethical reasons, but more because it could make the South rich enough to break away into a separate nation. Meanwhile the North was industrialising, and wanted to protect its industry through tariffs on imports, which would mainly harm the South.

Historians have compared the two sides to the Cavaliers and Roundheads of the English Civil War, two centuries earlier - from whom many of the Confederate and Union families were respectively descended.

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While this is what is taught in school, it is a lie. The Rothschilds plainly stated in the newspapers of the day that if Lincoln did not bow the knee to their usury, he "would find himself in a most unfortunate civil war."