The most important cause of the political debate over states' rights in the first half of the 1800s was the issue of slavery. The question of whether new states being admitted to the Union should allow slavery or not created a deep divide between the North and the South. The South believed that states had the right to determine their own policies regarding slavery, while the North believed that the federal government had the power to regulate slavery. This issue eventually led to the American Civil War and the end of slavery in the United States.