The primary (i.e., immediate) cause of the Civil War was the secession of the Deep South states, and the decision of the Confederate government to fire on Fort Sumter in Charleston Harbor. But I would argue that the political issue of slavery's expansion was the most important factor that led to secession. The issue of expansion hit a crisis point in the aftermath of the Mexican War, when Southern states demanded that slavery be allowed in the Mexican Cession, particularly California, which was set to become a state in 1850. While the Compromise of 1850 temporarily forestalled immediate conflict, the issue raised its head again with the Kansas-Nebraska Act, which opened Kansas and Nebraska to at least the possibility of slavery through the implementation of popular sovereignty. One result of this law was that Kansas exploded in conflict between anti-slavery settlers and pro-slavery "border ruffians" who attempted to establish a pro-slavery constitution there. Another result of particular relevance to this question is that the Kansas-Nebraska Act led to the rise of a new party, the Republicans, whose main issue was opposition to the spread of slavery. Many events happened to drag the nation toward disunion and civil war, but the election of a Republican, Abraham Lincoln, to the office of President was the single event that precipitated the secession of South Carolina and the rest of the Deep South. So it certainly could be argued that the issue of slavery's expansion was a major factor in driving the nation toward the Civil War.