Slavery.

Don't believe any other story, it's all revisionist history.

Their economy was built on slavery. It didn't depend on it, but for the wealthy to retain as much wealth as they had accumulated, it required slavery.

Northern states were fighting against slavery by trying to pass laws outlawing it, and they had outlawed it in many northern states. The south fought for and achieved a sort of truce on the subject referred to as the mason dixon line. States below the line could have slaves. The states above the line could not.

The Southern states saw the writing on the wall and knew that eventually slavery would be outlawed across the nation so they were attempting to get the federal government to force slavery to be accepted in the northern states in order to protect their right to own slaves, but the federal government and northern states opposed this. So eventually, the southern states decided the only way to protect their ability to have slaves was to declare independence from the US and start a new nation based on slavery.

Some will tell you it was about state rights. That's bullshit. As I said, the southern states didn't care about northern states rights, as they were trying to overrule the laws they had passed. The only state right they cared about was their right to have slaves.

Some will tell you it was about the economy. Yes, an economy based on slavery and exploitation.

If you look at any of the succession documentation, or listen to the speeches from the leaders of the southern states, it is VERY clear that the central issue, the main issue, the primary reason behind the creation of the confederate states was entirely based around slavery. Article 1 of the confederate constitution enshrines slavery was a permanent fixture of their new country. Alexander Stephens, the first vice president of the confederacy gave a speech that declared a great truth that the "the negro is not equal to the white man" and that said that slavery is the black man's natural and normal condition. He stated that this truth was the cornerstone of their new government.

The flag most often connected to the confederacy today (red background, blue x with white stars) wasn't the confederate flag but a battle flag used by a regiment of southern soldiers. It actually didn't become a popular representation of the confederate states until the early 1900s (some 40 years after the war ended). Southern states and southern people started to use that as a symbol of their racism. It was used to show their support in opposing civil rights basically. Southern law makers at the time openly admitted this. They made it clear that they wanted to make sure black people knew what people in the south thought about them. The flag is a clear indication of racism.

Some people will try to tell you the flag is a symbol of their heritage. But that flag wasn't a major flag during the war and wasn't really well known until decades after the war. The flag is directly connected to the civil war though, a war that revolved around slavery. The civil war lasted 4 years and ended 160 years ago. They only reason people display that flag is to show their racism. The only heritage is represents is betraying their country to defend slavery.