‘It was legal and would have remained legal in the USA but for the war.’
o why did the eleven states of the confederacy secede, and why did four of those eleven issue elaborate declarations of cause specifying the retention of slavery being threatened...? I mean, if it was legal and would have remained so absent the war, what were the secessionists afraid of...?
At least you got this part right. Most people think all 11 states issued these statements, but it was at most 4.
But I have three possible answers to your point.
1. Because they were somehow misled into believing that slavery was under a threat of being abolished if they remained.
2. Because claiming slavery as a cause for separation made separation legal on the grounds of breach of contract. (This idea was put forth by conservative columnist Paul Craig Roberts.)
3. To disguise their real reason for doing it because that would have provoked an overwhelming desire to stop them.
Which particular one would you like to focus on? I personally favor #3.
It could also be all of the above.
4 states issued declarations of causes. 3 of those 4 listed reasons (tariffs, unequal federal government expenditures, malicious refusal to provide border security) other than just that the Northern states had violated the compact by refusing to enforce the fugitive slave clause of the constitution. The Upper South seceded only after Lincoln chose war. They obviously were not seceding over slavery.
What is the first thing Lincoln and the Northern Dominated Congress offered? The Corwin Amendment which would have expressly protected slavery in the constitution. Given there were 15 states that still had slavery, and it takes 3/4s of the states to pass an amendment, it would have taken 45 states to vote in favor of overturning the Corwin Amendment. ie it could not be revoked without the consent of the states that still had slavery. Everybody understood this.
Yet, the original 7 seceding states turned down the offer of the Corwin Amendment as the price of their return. Obviously, protection of slavery was not their primary concern.