American Civil War
In the 1850s AD, white people in the southern part of the United States were getting more and more angry with rich people who lived in the North. One reason was that these northern rich people were getting richer from new factories they were building, and the southern rich people were not.
In 1860, the men who could vote elected Abraham Lincoln to be president (but no women, or people who had recently arrived in the United States, or Native Americans, or enslaved African-Americans were allowed to vote). Lincoln was a northerner, and he saw things in a Northern way. This made the southern people so angry and afraid that they decided to split off from the United States and form their own country, which they called the Confederacy.
Here's a good video explaining major issues of the Civil War
The states of Virginia, North and South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Texas, Arkansas, and Tennessee were in the Confederacy, and parts of Missouri and Kentucky. Mostly these were the states where it was legal to own slaves, but there were four states where slavery was legal that decided not to join the Confederacy.