From wikipedia you will find the following- The Union states were: * Border states: In Kentucky and Missouri, pro-secession factions declared for the South and those states were claimed by the Confederacy, but Unionist state governments remained in power. Kansas joined the Union on January 29, 1861,[2][3] after the secession crisis had begun but before the outbreak of fighting. West Virginia separated from Virginia and became part of the Union during the war, on June 20, 1863. Nevada also joined the Union during the war, becoming a state on October 31, 1864.