It is considered fundamentally wrong in America to challenge the reasons and outcome of what is most commonly referred to as the American Civil War. To do anything other than praise Abraham Lincoln as one of the greatest leaders of American history and the man who eliminated the horrible institution of slavery will get you thrown out of most history classes and censured if you happen to be part of the government.
Slavery, was in fact, the most major issue of contention, an institution that had existed for nearly a century before the war. The industrialization of the north allowed for slavery to be phased out earlier as it became economically feasible in the North. The Industrial Revolution had allowed the North to phase out slavery over decades. In the South, which continued to function primarily on agriculture for economic viability, it was not easy to suddenly free slaves and eliminate the horrible and inhumane owning of one's fellow man. While progress was being slowly made, it was too slow for vocal abolitionists, who had the ear of many in positions of government power in the North. The main point of contention was in the expansion of slavery to new territories and states. Attempts at appeasement, such as the Missouri Compromise, made matters worse by trying to keep everyone happy by maintaining a balance of "slave states" and "free states."
The growing cultural distance between the northern states and the southern states effectively divided the country into two halves long before secession. These compromises on new territories and states were made in the interests of maintaining balance in Congress. The South began to grow tired of having the North dictate standards, and with growing righteousness from radical, militant abolitionists gaining greater power in the North, the distance grew wider. With the election of Abraham Lincoln to the presidency, the division was complete. He had won the election without a single electoral vote from the southern states and very little of the popular vote. He was elected by the North, and regardless of where he stood on any issue, the South felt they had completely lost control of the government.
The real question was, "Did the South have the right to secede from the Union?" This was the issue the war was fought over. Given that the South felt they had lost all power in the United States government after Lincoln was elected, they felt within their rights to secede and form their own nation rather than allow the North to dictate to them. The majority view in the North was that the Union needed to be preserved at all costs and that accepting secession would pave the way for any group or minority element to leave the country at will.
The simplified view of the conflict, the one most often taught is: "Lincoln preserved the Union and eliminated slavery and saved the country from ruin." The result was a war that cost more American lives than any other, considering both sides were Americans. The question we are generally not allowed to ask, if it would have been better for all concerned if the South was allowed to secede, is an important one. Some have philosophized that the American South would have had serious economic problems "going it alone" and that necessary trading partners in Europe would have put heavy pressure on it to eliminate slavery, especially if they had ties to the American North. Without help from European states, the American South's one hole card, King Cotton, would have been meaningless.
Trying to build a nation in the 19th century without much of an industrial infrastructure and an unstable financial base would have forced the American South to ask for aid. The most obvious source of this aid would have been the American North. Thus, in order to survive, the American South would have had to take steps to eliminate slavery anyway. It is likely that eventually the American South would have reunited with the American North after a relatively short period of independence, or at the very least would have become a close ally. The sudden purge of slavery, that sent thousands of African Americans looking for work without any marketable skills in a racially hostile climate, which may or may not be resposible for racial tensions that continue today, might have been eliminated. Had diplomatic pressure and the demands of trading partners forced the American South to implement a phasing out of slavery, including the employment of former slaves rather than the owning of said slaves, the granting full citizenship and all associated rights, along with allowing them to seek other opportunities, who knows what may have developed. Was the preservation of the Union worth the lives of 620,000 Americans? I suppose it depends on who you ask.
Sources: Battle Cry of Freedom: The Era of the Civil War by James M. McPherson (fact checking)
My own fragile, eggshell mind (opinions and extrapolations)
For the record, I prefer War Between the States as the name of the conflict
PS: I was born in New York and lived most of my life in Massachusetts