Abraham Lincoln was elected president. He decided he wanted to get rid of slavery, but some people from the south of America didn't like that.
Alabama, Florida, Georgia, Louisiana, Mississippi, South Carolina, and Texas all left the union because of this. California, Connecticut, Delaware, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland and 13 others stayed loyal.
After Lincoln's failed attempt to bring the Confederate states back and refusal to remove union troops from the Confederacy in 1861, South Carolina attacked Fort Sumter.
This was the motivation for the Union Troops to start being sent to regain the south states. War has begun.
After the battle of Chattanooga, Tennessee was captured and brought back into the union. Then, Atlanta, Georgia was seized and burned.
The union passed the thirteenth amendment; "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction. Congress shall have power to enforce this article by appropriate legislation."