[ruined]

This document was simultaneously published by the very committee that issued the initial ordinance of secession in South Carolina, and it explains the decision to secede at great length. It is all about slavery:

https://en.wikisource.org/wiki/The_Address_of_the_People_of_...

Additionally, articles of secession from other states also explicitly mention slavery in the actual text of the law, including Alabama's ordinance that proposed the Montgomery Convention at which the Confederacy was founded. Even more states explain it in similar justifying documents.

The vice president of the Confederacy delivered a speech establishing slavery as the primary motivator of secession: https://en.wikipedia.org/wiki/Cornerstone_Speech

Jefferson Davis, president of the Confederacy, is on record before the war explaining slavery as cause, and after the war claiming it was about states' rights (to practice slavery).

[s1artibartfast]

This whole debate is semantics and pedantics about the word cause. The reality is that it doesn't make sense to reduce history to a single factor.

The south wanted to secede because the union was threatening their economic interests which relied on slavery. The union did not want the south to secede for their own reasons, of which slavery was not the foremost.

You can not remove any part of that explanation and still describe the "cause" of the war.

The war would not have happened if slaves didn't have economic value. The war would not have happened if the north was indifferent to the south leaving.

Trying to reduce things further than this is a fools errand.

Your linked document is a great example of this. It discusses a great many things. The first half is a history of grievances about states rights and failures of the union to uphold the constitution.

It starts to talk about slavery about half way through if you ctrl-f.

Anyone arguing for their singular preferred cause can find supporting evidence in it because the "cause" of the war was the interaction of multiple factors, so evidence exists for all of them.

The problem is that showing evidence that one factor was critical to the start of the war does not prove that other critical factors do not exist.