@ Didge:
I don't think the philosophical or moral question of slavery was the prime cause, but the plantation owners in the South had, or thought they had, a tremendous economic stake in slavery, having overlooked the fact that with industrialization and mechanization, slavery was becoming obsolete. Nevertheless, they had been doing their best to protect slavery for decades, with political compromises about which newly-admitted states to the Union would permit slavery and which would not, based on the fear that eventually slavery would be voted out of existence. There were also other economic issues, the North favoring tariffs on imported manufactured goods (to encourage domestic manufacturing), and the South opposing tariffs.
Lincoln himself summarized it very well in his second inaugural address of March, 1865:
" One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union even by war, while the Government claimed no right to do more than to restrict the territorial enlargement of it. Neither party expected for the war the magnitude or the duration which it has already attained. Neither anticipated that the cause of the conflict might cease with or even before the conflict itself should cease."