This is not just a question of what happened, but also how things are remembered. Let’s look at what happened.

Hank Green’s high school teacher responded to the claim that the Civil War was over state’s rights by asking, “A state’s right to what sir?” In short, the right to have slaves. There had been previous conflicts over the division of power between the federal and state governments, such as the struggle over the Tariff of 1832, which led to the Nullification Crisis of 1833. That crisis was led by South Carolina, which would also lead the Secession Crisis resulting in the civil war. Both these events were articulated as questions of states’ rights, and are the roots of what many people continue to see as the reason for the Civil War. States’ Rights was the legal argument justifying the actions of the states.

This is a very important point that is often missed: The justification for war and the reasons for the war are rarely the same. The term casus belli is a very old term for the legal justification for going to war. There is usually a need to justify a war, whether you have to convince your citizens to support the war or to prevent other powers from getting involved. Some great examples of this distinction, besides the American Civil War, are the wars of German Unification. If you are interested, look at why Otto von Bismarck wanted to have the wars, and the justification he presented to the Prussian king, the German people, and the other powers of Europe. Look at the Schleswig-Holstein Crisis, the lead up to the Austro-Prussian War, the Ems Telegram, and the Franco-Prussian War.

In the cases mentioned above, the reason Bismarck wanted to go to war was to unify the German states by any means necessary, but simply declaring war and conquering them would have resulted in a massive backlash from other European powers trying to preserve the balance of power that had emerged after the Napoleonic Wars. He had to convince them that his wars were justified, or he would have ended up like Napoleon.

So why am I talking about European wars to answer a question about the American Civil War? To accurately understand the motivations behind the war, we need to look behind the legal justification and public rhetoric offered, and look for people’s actual reasons. Unsurprisingly, these reasons are largely, but not wholly, economic. The South’s economic system was based on slave labor and trade relations with other countries. The Tariff of 1832 threatened that by massively increasing the tax, in the form of a protective tariff, on goods imported from other countries. The idea behind the tariff was to protect the developing industry in the North from competition from other countries. The southern Economy traded cotton for goods from Britain. These goods tended to be cheaper and of better quality than the competitors in the North. As a result, the burden of the tariff fell most heavily on Southern slave owners. The Nullification Crisis was resolved peacefully and established the legal precedent of federal rights being higher than state rights. This was articulated in the Constitution and was the primary reason for repealing the Articles of Confederation in favor of the Constitution, but this was the first serious challenge to it.

In the lead up to the Civil War, there were also a series of conflicts over whether states would be admitted as slave-owning or non-slave owning. These included the Compromise of 1850 and the Kansas-Nebraska act. The Slave owning states were trying to maintain enough seats in congress that they could prevent laws like the Tariff of 1832 that hurt or challenged the economic system they were dependent on. That system was slave labor. During the 1860 presidential election, many believed if the Republican candidate won, they would not be able to continue to admit states as slave-holding states. When Lincoln won the election, there was little doubt in their minds that he would act to prevent new slave states being admitted. The Southern states wanted to maintain slavery as their state’s social and economic system. They claimed that each state had the legal right to secede from the Union and that they were just exercising that right. Lincoln argued that no such right existed under the constitution. These were their legal arguments, but they were not the reasons why they were making these arguments.

TL;DR: Southern states were attempting to ensure the continuation of slavery, and were stating a legal argument grounded in the idea of state’s rights to justify their action.

Edit: Grammar