Prior to the American Civil War, the issue was of slavery was still very controversial both in the North and the South. The real issue between the late 1830s and mid-1850s was the expansion of slavery in newly formed territories and states. The problem was caused by the Three Fifths Compromise of 1787, as it stated that all slaves counted as three-fifths of people when the census was taken in order to determine representation in the US Legislature.
The South didn’t want more free states or territories added to the union, as that would dilute their pro-slavery caucus, and mean more pressure to end the institution of slavery. The arguments between the North (mostly pro-abolitionist) and South (mostly pro-slavery) continued for decades prior to the outbreak of actual fighting in the early 1860s.
This all culminated in the election of Abraham Lincoln in 1860. For years, Lincoln was a staunch abolitionist (though that is not a term he agreed with). There are so many quotes from Abe Lincoln about this issue it is hard to choose which one to present here. Like this one from a speech he gave to an Indiana Regiment in 1865: “Whenever I hear any one arguing for slavery I feel a strong impulse to see it tried on him personally.” Or perhaps this one from years before the war: “In the first place, I insist that our fathers did not make this nation half slave and half free, or part slave and part free. I insist that they found the institution of slavery existing here. They did not make it so, but they left it so because they knew of no way to get rid of it at that time.” (From the 1858 Lincoln-Douglas Debate).
The one that most captures the conflict prior to the Civil war is this one from a speech he gave in 1859 in Chicago, Illinois: “”I do not wish to be misunderstood upon this subject of slavery in this country. I suppose it may long exist, and perhaps the best way for it to come to an end peaceably is for it to exist for a length of time. But I say that the spread and strengthening and perpetuation of it is an entirely different proposition. There we should in every way resist it as a wrong, treating it as a wrong, with the fixed idea that it must and will come to an end.”
By the time 1860 rolled around, the Civil War was inevitable from the South’s point of view, while the North (and Lincoln himself) still thought there were peaceful solutions to the problem. In 1860, when South Carolina became the first of the Southern states to secede from the Union, there was no choice but for both sides to fight the war on the battlefield.