Answer:
Slavery
Step-by-step explanation:
The moment leading up the civil war, the Federal government forced the southerners to end the practice of slavery and exchange it with machines/industries.
The southerners did not want to do it since their economy at that time was heavily reliant on slave labors.
So, the southern states decided to secede. They separated themselves from the Federal government's rule with the intention to form new independent nations.