It is not an oversimplification to say that slavery was the single most important issue leading to the Civil War. For not only was slavery the most important on its own merits, but none of the other relevant issues, such as expansion into the western territories or states’ rights, would have mattered much at all if not for their indelible connection to slavery.
Initially, Northerners rallied around the issue of Free Soil: opposition to slavery on economic grounds. Small farmers and new industrial workers did not want to compete with large slave plantations and unpaid slave labor. This was the philosophy that bound together the new Republican Party.
No friends of African Americans, most Free Soilers were openly racist, as were the vast majority of white Americans at the time. Abolitionists, who were fired by religion and opposed human bondage on moral grounds, were actually a small minority of the population However, as the bloody war raged on, Northerners began to seek moral assurance in their cause. For more and more people, the mere political goal of saving the union did not seem to justify the unholy slaughter of men by the tens of thousand. Though preserving the union was always Abraham Lincoln’s primary goal, he astutely played to this concern by issuing the Emancipation Proclamation and establishing abolition as the war’s moral compass. It worked. The North persisted, won the war, abolished slavery, and forced the South to return.