In the year 1860 Abraham Lincoln, an antislavery republican representing the North, won the presidential election by having the majority in electoral votes. This angered the whites in the slaves states in the South whom were already contemplating seceding from the Union. They knew that with Abraham in the office the abolition of slavery was near and they were not going to allow that to happen; not without defending it first. Cotton was their "cash cow" and they used slave labor to produce the cotton so they saw themselves doomed without slavery. So, instead of doing the hard labor themselves or hiring people to do it for them, they were like, oh hell no we'd rather separate ourselves from the Union, ruin the future of America and kill a couple thousand people to defend our right to enslave humans. This was very disappointing, after everything the forefathers and the citizens of America went through just to become a nation they just were willing to let it fall apart because they were lazy and cheap!
Now back to the story at hand, South Carolina was the first slave state to chuck it all away and secede from the Union, then Mississippi soon after. Over the next six weeks Alabama, Florida, Georgia, Louisiana, and Texas followed suit and seceded from the Union and became the Confederate States of America. Both Abraham Lincoln and Jefferson Davis, president of the Confederacy, were trying to find non violent ways to solve this crisis without looking soft to their angry citizens griping at them about what they want done. After a while both presidents were considering war but the upper South, the slave states that didn't secede, still wanted the chance at solving this with negotiation. They tried passing an amendment that protected and guaranteed slavery in the states where it already existed, but the Republicans said no deal. The deep South states tried to convince their slave holding brethren in the upper South to join the cause and secede from the Union by sending representatives over to talk to them letting them know that war was coming and they need to stand together as one.
April 12, 1861, the deep South decided that it was time to stop talking and show the North that they mean business and attacked an unarmed ship carrying food and supplies for the soldiers at Fort Sumter. The North took this as a declaration of war and enlisted 75,000 men and got mobilized. For some reason the upper South states saw this as the North betraying them, like really, the South shoots up an unarmed ship and the North is in the wrong? How did they even come to that conclusion? So four slave states: Virginia, North Carolina, Tennessee, and Arkansas caught hurt feelings and seceded to the Confederacy. The remainder of the slave states: Missouri, Kentucky, Maryland, and Delaware; decided to remain loyal to the Union.