The Civil War was fought between the United States Federal Government, the Union, and the Confederate States of America, the Southern slave states. The Union was led by President Abraham Lincoln and the Confederacy was led by President Jefferson Davis. The Union opposed the expansion of slavery and the right of Confederate states to secede. The Confederacy claimed a right to form an independent Confederate nation supportive of slavery.

The Union states consisted of: California, Connecticut, Delaware, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, Ohio, Oregon, Pennsylvania, Rhode Island, Vermont, and Wisconsin. Nevada and West Virginia joined the Union as new states during the war. Tennessee and Louisiana returned to Union control early in the war. The Colorado, Dakota, Nebraska, Nevada, New Mexico, Utah and Washington territories fought with the Union.

South Carolina, Mississippi, Florida, Alabama, Georgia, Lousiana and Texas seceded and formed the Confederate States on February 1861.