This post originally appeared on the Guardian’s website.   One hundred fifty years ago, on 20 December 1860, the South Carolina secession convention officially dissolved the state’s connection with the American Union. The secession of South Carolina set in motion a crisis that culminated in four years of civil war, the preservation of national unity and the destruction of the largest slave system the modern world has known.

Contemporaries had little doubt about the reasons for secession. With no support in the slave states, the Republican party had just elected Abraham Lincoln president on a platform committed to halting slavery’s westward expansion. Lincoln himself had called slavery a "monstrous injustice" and had declared that the nation could not exist indefinitely half-slave and half-free. In explaining its decision, South Carolina’s convention warned that the ultimate result of Republican rule would be "the emancipation of the slaves of the South."

Within a few months, 10 slave states had joined South Carolina in the Confederate States of America. Its founders forthrightly announced that they had created a slaveholders’ republic. The new nation’s "cornerstone," declared Confederate Vice-President Alexander H Stephens, was the principle "that slavery, subordination to the superior race" was the "natural and moral condition" of black Americans.

Four years later, in his second inaugural address, Lincoln would affirm that slavery was "somehow" the cause of the war. This is now an unquestioned axiom among historians. Yet, many Americans today resist this basic truth. They insist that differences over other issues—states rights, the tariff, constitutional interpretation—led the nation into war.

What does it mean to say that slavery caused secession and the war? Not that the South was evil and the North moral. In his second inaugural, Lincoln spoke of "American," not southern, slavery—his point being the complicity of the entire nation in the sin of slavery. Few northerners demanded immediate abolition. Abolitionists were a small and beleaguered minority. Sectional differences certainly existed over economic policy, political power and other matters. But in the absence of slavery, it is inconceivable that these differences would have led to war.

Rather, it means that by 1860, two distinct societies had emerged within the United States, one resting on slave labour, the other on free. This development led inexorably to divergent conceptions of the role of slavery in the nation’s future. Northern Republicans did not call for direct action against slavery where it already existed—the constitution, in any event, made such action impossible. But Lincoln spoke of putting slavery on the road to "ultimate extinction," and he and other Republicans saw his election and a halt to the institution’s expansion as a first step in this direction. Secessionists saw it this way as well.

A century and a half after the civil war, many white Americans, especially in the South, seem to take the idea that slavery caused the war as a personal accusation. The point, however, is not to condemn individuals or an entire region of the country, but to face candidly the central role of slavery in our national history. Only in this way can Americans arrive at a deeper, more nuanced understanding of our past.