Border states in the American Civil War were states that permitted slavery but did not leave the Union to join the Confederacy. The states recognized by both sides as border states were Kentucky, Maryland, Delaware, Missouri and West Virginia.
The border states were courted to join the Confederacy during the conflict because of their geographic importance, but all five chose to remain in the Union. Many battles were fought in Kentucky, and Missouri was a hotbed of guerrilla warfare during the war. The famous Emancipation Proclamation only freed the slaves in Confederate states and did not apply to slaves living in the border states. Those slaves were not actually freed until after the war and the passage of the 13th Amendment to the U.S. Constitution.