Historical military map of the border and southern states by Phelps & Watson, 1866
Map of the division of the states during the Civil War. Blue represents Union
states, including those admitted during the war; light blue represents border states; red represents Confederate
states. Unshaded areas were not states before or during the Civil War.
In the context of the American Civil War (1861–65), the border states were slave states that did not declare a secession from the Union and did not join the Confederacy. To their north they bordered free states of the Union and to their south they bordered Confederate slave states. Of the 34 U.S. states in 1861, nineteen were free states and fifteen were slave states. Two slave states never declared a secession or adopted an ordinance: Delaware and Maryland. Four others did not declare secession until after the Battle of Fort Sumter and were briefly considered to be border states: Arkansas, North Carolina, Tennessee, and Virginia—after this, they were less frequently called “border states”. Also included as a border state during the war is West Virginia, which was formed from 50 counties of Virginia and became a new state in the Union in 1863.
In the border states there was widespread concern with military coercion of the Confederacy. Many, if not a majority, were definitely opposed to it. When Abraham Lincoln called for troops to march south to recapture Fort Sumter and other national possessions, southern Unionists were dismayed. Secessionists in Arkansas, North Carolina, Tennessee, and Virginia were successful in getting those states to declare their secession from the U.S. and to join the Confederate States of America.
In Virginia, Kentucky, and Missouri, there were both pro-Confederate and pro-Union governments. West Virginia was formed in 1862–63 after Virginia Unionists from the northwestern counties of the state then occupied by the Union Army, had set up a loyalist "restored" state government of Virginia. Lincoln recognized this government and allowed them to divide the state. Kentucky and Missouri had adopted secession ordinances by their pro-Confederate governments (see Confederate government of Kentucky and Confederate government of Missouri), but they never fully were under official Confederate control, though at various points Confederate armies did enter those states and controlled certain parts of them. Though every slave state except South Carolina contributed white battalions to both the Union and Confederate armies (South Carolina Unionists fought in units from other Union states), the split was most severe in these border states. Sometimes men from the same family fought on opposite sides. About 170,000 border state men (including African Americans) fought in the Union Army and 86,000 in the Confederate Army.
Besides formal combat between regular armies, the border region saw large-scale guerrilla warfare and numerous violent raids, feuds, and assassinations. Violence was especially severe in eastern Kentucky and western Missouri. The single bloodiest episode was the 1863 Lawrence Massacre in Kansas, in which at least 150 civilian men and boys were killed. It was launched in retaliation for an earlier, smaller raid into Missouri by Union men from Kansas.
With geographic, social, political, and economic connections to both the North and the South, the border states were critical to the outcome of the war. They are considered still to delineate the cultural border that separates the North from the South. Reconstruction, as directed by Congress, did not apply to the border states because they never seceded from the Union. They did undergo their own process of readjustment and political realignment after passage of amendments abolishing slavery and granting citizenship and the right to vote to freedmen. After 1880 most of these jurisdictions were dominated by white Democrats, who passed laws to impose the Jim Crow system of legal segregation and second-class citizenship for blacks. However, in contrast to the Confederate States, where almost all blacks were disenfranchised during the first half to two-thirds of the twentieth century, for varying reasons blacks remained enfranchised in the border states despite movements for disfranchisement during the 1900s.
Lincoln’s 1863 Emancipation Proclamation did not apply to the border states. Of the states that were exempted from the proclamation, Maryland (1864), Missouri (1865), Tennessee (1865), and West Virginia (1865) abolished slavery before the war ended. However, Delaware  and Kentucky did not abolish slavery until December 1865, when the Thirteenth Amendment was ratified.