Sectionalism: excessive regard for sectional or local interests; regional or local spirit, prejudice, etc. (Dictionary.Com)
My definition: when one section of the country believes it is better than the other (they thought their opinion on tariffs, slavery, etc. was the only right one.)
First of all, states' rights could have been a part of sectionalism. Each side, the and the south, thought that what they believed about states' rights was correct: the union wanted a stronger central government, and he south wanted the states to have more rights than the central government. Additionally, sectionalism is a more vague topic and covers many other conflicts before the Civil War. For example, the north and the south fought over whether to have high tariffs or not. This conflict was one of the many that led to the Civil War; the north thought they should have higher tariffs, while the south believed in lower ones. Overall, sectionalism was the main cause of the civil war because if the north and the south didn't fight over their different opinions, there would not have been reason for war in the first place.
The main cause of the American Civil War was slavery. Every other issue is derivative from that central issue. It sectionalism… about the issue of slavery. It was state’s rights… concerning slavery. People fought in the American Civil War for a variety of reasons, but the war itself was about slavery.
Although they had a great deal to do with the war, it would come down to a division of what states were allowed to do, and what the government could do to stop them.
The southern states felt as though they were being treated unfairly, and threatened to leave the newborn United States in protest. This precedent would be disastrous and embarrassing for the US, and would make it impossible for us to continue as a stable, unified entity.
While there were many factors at play, the largest was simply this:
The federal government laid down the rules.
The south attempted to leave the union rather than comply with said rules.
The union took the rebellious faction by force and pretty much removed the choice of secession.
Differences in economy were the first symptoms of trouble ahead. The Southern economy was largely dependent on cotton, and therefore on slavery as well, whereas the North was developing industrially. On top of that, and the main reason being, the states feared the federal government was attempting to impede their rights, especially that of nullification.
Slavery was a component of the war, but more of an "after the fact" one.
The US Civil War was about slavery. Clear and simple, cut and dry. There were discussions about sectionalism and state's rights that emerged from the desire of southerners to keep their slaves, but on a basic level, it was about slavery. Anybody who objects is simply disregarding reality and knowingly obfuscating the truth.