The border states, which had not seceded, but separated the United States from the Confederate States, were of great importance to the Northern war strategy. In order to win the war, the Union Army would need to pass through the border states to invade the South and, because of Confederate sympathies, there was Northern concern over marching through potentially hostile territory. The border states also contained the greater portion of the South's food and fuel and three-quarters of its industrial capability.
The four original border states, all of which remained slave states at the onset of the war in 1861, were Delaware, Kentucky, Maryland and Missouri. Delaware remained loyal to the Union and Kentucky claimed neutrality. Because Delaware controlled access to the city of Philadelphia and Kentucky's northern border stretched for 500 miles along the Ohio River, these two states were of great strategic and logistical value to the North.
Maryland represented a particular threat to a Northern victory because it surrounded the Union capital and a significant portion of its population supported the Confederate cause. Although Maryland remained part of the Union, it did so under duress. U.S. President Abraham Lincoln suspended the writ of habeas corpus and declared martial law. This enabled Union troops to detain secessionist leaders and Confederate sympathizers without due process. The fourth border state of Missouri was persuaded to remain part of the Union by an uneasy occupation of the state by Northern troops.
By 1863, an additional border state was created when the northwestern portion of the Confederate state of Virginia refused to support the secessionist cause and broke away to form the state of West Virginia. The new state was officially received into the Union through a statehood bill enacted by Congress and signed by President Lincoln.