"A house divided against itself cannot stand."

When Nikki Haley seemed not to know the cause of the American Civil War, that whole argument re-emerged. The answer to the question is simple: slavery caused the Civil War. James M. McPherson, author of Battle Cry of Freedom, explains why Civil War historians don't consider it an open question:

In the 186os, few people in either North or South would have dissented from Abraham Lincoln's statement, in his second inaugural address, that slavery "was, somehow, the cause of the war." After all, had not Jefferson Davis, a large slaveholder, justified secession in 1861 as an act of self-defense against the Lincoln administration, whose policy of excluding slavery from the territories would make "property in slaves so insecure as to be comparatively worthless ... thereby annihilating in effect property worth thousands of millions of dollars"? And had not the new vice president of the Confederate States of America, Alexander H. Stephens, said in a speech at Savannah on March 21, 1861, that slavery was "the immediate cause of the late rupture and the present revolution" of Southern independence? The old confederation known as the United States, said Stephens, had been founded on the false idea that all men are created equal. The Confederacy, by contrast, "is founded upon exactly the opposite idea; its foundations are laid, its cornerstone rests, upon the great truth that the negro is not equal to the white man; that slavery, subordination to the superior race, is his natural and moral condition. This, our new Government, is the first, in the history of the world, based on this great physical, philosophical, and moral truth." ....

Slaves were the principal form of wealth in the South. The market value of the four million slaves in 1860 was close to $3 billion dollars—more than the value of land, of cotton, or of anything else in the slave states. Slave labor made it possible for the American South to grow three quarters of the world's marketed cotton, which in turn constituted more than half of all American exports in the antebellum era. But slavery was much more than an economic system. It was a means of maintaining racial control and white supremacy. The centrality of slavery to "the Southern way of life" focused the region's politics on defense of the institution. But it was not the existence of slavery that polarized the nation to the breaking point, but the issue of the expansion of slave territory. Most of the crises that threatened the bonds of union arose over this matter. The first one, in 1820, was settled by the Missouri Compromise, which balanced the admission of Missouri as a slave state with the admission of Maine as a free state and banned slavery in the rest of the Louisiana Purchase territory north of 36°30' while permitting it south of that line. Paired admission of slave and free states during the next quarter century kept their numbers equal. But the annexation of Texas as a huge new slave state—with the potential of carving out several more within its boundaries—provoked new tensions. It also provoked war with Mexico in 1846, which resulted in American acquisition of three-quarters of a million square miles of new territory in the Southwest. This opened a Pandora's box of troubles that could not be closed.

James M. McPherson, “What Caused the Civil War?,” North & South: The Official Magazine of the Civil War Society (pdf)