The Border states reference article from the English Wikipedia on 24-Apr-2004
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Border states

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In histories of the American Civil War, the border states were those whose laws condoned slavery but that did not secede. They were Maryland, Delaware, West Virginia, Kentucky and Missouri. West Virginia was formed in 1863 from the northwestern counties of Virginia that had seceded from Virginia after Virginia seceded from the Union. In the cases of Kentucky and Missouri, the states had two state governments during the American Civil War, one supporting the Confederacy and one supporting the Union.

With geographic, social, political and economic connections to both the North and South, the border states were critical to the outcome of the war and still delininate the cultural border that separates the North from the South.