I believe that the road to war was paved with mistakes and miscalculations. I also think that the Republicans and the Southern Democrats had fallen under the control of the extreme wings of their respective parties. Neither side, primarily for political reasons, was able to negotiate with or compromise due to the unbending views of their constituency. I guess one might say war was inevitable due to the fact that the "Radical Abolitionists" and "Fire Eaters" appeared to be in control, leading up to the war.
If I look at any single event that led to war, it was the splitting of the Democratic party in 1860, which allowed the election of a minority president in Abraham Lincoln. I will always wonder how the U.S. would have settled the slavery issue with Stephen Douglas/Herschel Johnson as president? With a Douglas win in 1860, would Lincoln have been a viable candidate in 1864 or 1868?
Of course, we'll never know that alternative history or how different the U.S. might be today with a peaceful solution of the slavery question.