Abolitionists started trying to make slavery illegal in the mid-1700s. By 1804, all of the northern states had ended slavery. However, none of the Southern states had. The Southern states believed that slavery was their right, and they did not want to give it up. Cotton had become a very important crop in the South. Owners of large cotton plantations were used to having slaves to do work for free, which made the plantation owners richer because they did not have to pay anybody to work.