Abraham Lincoln said in his major speech he would preserve slavery if it preserved the Union.
Lincoln was a white supremacist, who believed blacks inferior. Lincoln's plan was to "deport" or send back to Africa all blacks, to keep America all white. Lincoln married into a major slave owning family. Lincoln chose a slave owner Andrew Johnson as his running mate. Lincoln chose a slave owner as his commanding general: Grant. Lincoln as an attorney had represented a slave owner trying to return a runaway slave from Illinois to Kentucky. Lincoln's speeches were impressive and mislead endless historians. Lincoln does appear "enlightened" compared to Douglas who believed blacks were subhuman.
Lincoln was a corporate lawyer, into money. As president he was more interested in financially dominating the South than freeing slaves. He "evolved" during the Civil War to appeal to antti-slavery sentiment in Britain, only because he feared Britain would enter the Civil War on the side of the South, due to commercial reasons.
Robert E. Lee opposed slavery and proposed to Jefferson Davis emancipating blacks if they served in the Confederate military. Davis declined. Lee said had he known the war was over slavery he would have served for the North. Grant said had he known the war was over slavery, he would have served in the Confederate military.
Tariffs were the major issue, as the South rebelled against major financial taxation by the North. Just as the colonialists rebelled against Britain in 1776 over taxation.
It makes good mythology to think it was a war over slavery. It's an after the fact theory to justify major bloodshed, the bloodiest war in history short of WW2. History is written by the winners.
Even Presidents Woodrow Wilson and Eisenhower defended Robert E. Lee as a patriot, as they argued back then it was not considered a closed issue whether states could secede. Even New England abolitionists had considered seceding over slavery, arguing for states' rights. "States" are nations. We were the United States, not the United Provinces.
Lincoln's "Emancipation" proclamation freed no one. It applied only to Confederate areas where it had no effect, while it kept blacks as slaves in Union held areas.
Ebony magazine, a black written article makes most all these same points, arguing its mythology to believe the war was over slavery.
Lincoln was a shyser/lawyer/politician into rhetoric, not substance. he would have been killed by abolitionists if John Wilkes Booth hadn't killed him. Grant later invited Lee to the White house. Lee was respected by the North as well as the South. Lee went on to run a university where he expelled any white student who disrespected a black, in the university or in the local town.
Most of the US opposed the war. Lincoln was only a plurality president and only won re-election by disenfranchising half the country, which would almost unanimously voted against him. Draft riots broke out in NYC.
but if this is a college course question, don't try arguing this, as it goes against what your instructor likely will ever believe. Instead read Fonier, and pretend to believe his theory that Lincoln "evolved" into a an ant-slavery president out of ethical concerns.
Unlike Lincoln, Jefferson Davis chose a non-slave owner, as his vice president.
even if you believe it was a war to end slavery, there is a view, such as Charles Beard's that economics is the reason for wars. Money seems to motivate wars more than idealism. Idealism or humanitarian goals may lead to war, but more likely only if there are financial incentives at the same time. IF there is major money to gain by going to war, idealist reasons mask that. After WWI, isolationism set in in America, as people believed bankers promoted involvement in the war, to make major profits. Michael Moore did his film arguing America went into Iraq for oil. It's rare to see a war fought purely on humanitarian grounds. They always seems to be some major financial motive, regardless how good the idealism or humanitarian motive.
John Stuart Mill wrote a piece on the American Civil War. Mill was very anti-slavery and supported the North as if the war was about ending slavery. Mill was British and may have been writing to keep Britain out of the war, as Britain had commercial interests in resuming trade with the South, hindered by the North's war and embargo on the South. Fonier is a major historian who although arguing the war was or became about slavery, at least recognizes Lincoln had past that was more racist than historians tend to acknowledge. Much of Lincoln history has that mythology aspect about it, like Lincoln walking 10 miles int the snow to return a penny he overcharged a customer.