Okay, so imagine if you and your best friend had a big fight and stopped talking to each other. That's what happened to a big group of states in America a long, long time ago. They got into a big fight about something called "slavery" and stopped talking to each other.
One group of states was called the "Union" and the other group was called the "Confederacy". The Union was like the good guys in a movie and they believed that slavery was wrong. The Confederacy believed slavery was okay.
The Union was led by a man named Abraham Lincoln who wanted to keep all the states together like one big family. He said that the Constitution of the United States didn't allow states to leave the Union.
So, they had a big war between the Union and the Confederacy. It was a scary time for everyone. Many people fought and died in the war. But the Union was the one who won the war, and all the states stayed together in one big family again.
And they all learned that it's important to talk and work together to solve problems instead of just fighting and causing a big mess.