The American Civil War lasted from 1861 to 1865. Divided over issues such as slavery and states' rights, the northern and southern states fought brutally against one another. The number of states that made up the southern, or Confederate, states increased just after the war began. The "Border States" allowed slavery, but never left the Union.

The Union

Twenty states made up the Union and included Oregon, Nevada, California, Iowa, Minnesota, Wisconsin, Michigan, Kansas, Illinois, Indiana, Ohio, New Jersey, Pennsylvania, Rhode Island, Connecticut, Massachusetts, Vermont, New Hampshire, New York and Maine. The Union population was 18.65 million, according National Park Service estimates.

The South

Once Abraham Lincoln, who was opposed to slavery, was elected in in 1860, seven states quickly seceded from the Union. They were South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana and Texas. Virginia, Arkansas, Tennessee and North Carolina followed shortly after the start of the war. The Confederacy included 5.5 million free people and 3.5 million slaves, according to the NPS.

The Border States

Five states elected not to secede from the Union; however, they still allowed slavery. Delaware, Kentucky, Maryland, Missouri and West Virginia were border states and were often places of great contention. Abolitionists and pro-slavery populations lived alongside one another, and this though this co-existence often led to heated confrontations. According to the NPS, the Border states had 2.5 million people and a half million slaves.