The Commonwealth of Virginia was the first State to outlaw the importation of slavery in the Union in 1832 I believe, although there was a general importation ban for all the States in 1807 to little effect.
As far as slavery being the cause of the war, if that were the case then why would Lincoln attempt to make a deal with the Southern States to protect slavery forever through a Constitutional Amendment known as the Corwin Amendment. Equally,if slavery had been the reason for the South's secession they could have simply returned to the Union and slavery would have been protected. The South could have completely avoided the devastation of war by simply accepting Lincoln's deal.
"We are not a nation, but a union, a confederacy of equal and sovereign States" John C. Calhoun
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