The U.S. state of West Virginia was formed out of western Virginia and added to the Union as a direct result of the American Civil War, in which it became the only modern state to have declared its independence from the Confederacy. In the summer of 1861, Union troops, which included a number of newly formed Western Virginia regiments, under General George McClellan drove off Confederate troops under General Robert E. Lee at the Battle of Philippi in Barbour County. This essentially freed Unionists in the northwestern counties of Virginia to form a functioning government of their own as a result of the Wheeling Convention. Before the admission of West Virginia as a state, the government in Wheeling formally claimed jurisdiction over all of Virginia, although from its creation it was firmly committed to the formation of a separate state.

During the American Civil War, the United States was referred to as simply the Union, also known colloquially as the North, after eleven Southern slave states seceded to form the Confederate States of America (CSA), which was called the Confederacy, also known as the South. The name the "Union" arose from the declared goal of the United States, led by President Abraham Lincoln, of preserving the United States as a constitutional federal union.