Why did slavery start the civil war

The Civil War started because of uncompromising differences between the free and slave states over the power of the national government to prohibit slavery in the territories that had not yet become.

President Abraham Lincoln declared that slavery would be illegal in the United States. The South States decided that they didn't like that since slavery was their main source of income, so they declared independence. Slavery was one of the main causes of the American Civil War, but it wasn't the only one.