Requesting some advice on a question

Would you mind explaining why you have biased emotions on the subject? It seems strange to me. Aside from being a 150-year-old conflict, the moral questions revolving around the Civil War seem like they pretty much swamp the legal questions. From the C.S.A's perspective even if secession was illegal I would still expect them to do it given what they believed and would consider them justified if their beliefs and values were mine. Same goes for the North. The law seems like a minor player here.

Questions of the form "Is it legal?" that appear hard are usually meaningless.
Even when they appear easy, they often conflate questions that have different answers.

Taboo "legal." Why do you care?

Legally, circa the time of the actual secessions leading to the creation of the C.S.A, was secession legal?

A bit of a side note, but "C.S.A." isn't an obvious acronym for a Frenchman like me, though I guessed what you were talking about from the context.

The U.S. Constitution says nothing at all about the issue of secession, and the question was controversial in the years before the Civil War. In retrospect, it seems crazy that states should form a union without specifying whether they'll be allowed to secede from it, but this is what actually happened, with the worst imaginable consequences following eventually. (Though it should be noted that the main focal point of the antebellum constitutional controversies was the issue of nullification) rather than secession.)

Needless to say, the seceding Confederate states considered that the states' right to secession was implicit in the U.S. Constitution, which was according to them an arrangement between sovereign states from which they reserved the right to withdraw at will. It's notable that the Confederate Constitution was largely a literal copy of the U.S. one, and the Confederate secessionists made a point by refusing to add anything more about the issue of secession into it.

On the other hand, the ruling precedent in the U.S. constitutional law was set by the post-Civil War Supreme Court decision in Texas v. White. Unsurprisingly, the court ruled that the states have no right to secession, so that the Confederate States had legally remained part of the U.S. throughout the Civil War and Reconstruction, and all the acts of secession and the subsequent legal acts of the Confederacy were thus null and void.

An act is illegal if the government punishes you for doing it.

In terms of what's written down on paper, I don't think it was mentioned at all, which implies legality. But, practically speaking, are governments really ever okay with part of their territory claiming they don't have to pay taxes anymore? I suspect people in the southern states were aware of the answer.

Wikipedia:Secession in the UnitedStates has a pretty good overview for the uninformed (like me).

Interesting bit; Madison saying, at the time of the constitution:

[...]this dodges the blow by confounding the claim to secede at will, with the right of seceding from intolerable oppression. The former answers itself, being a violation, without cause, of a faith solemnly pledged. The latter is another name only for revolution, about which there is no theoretic controversy.

... so the intent of the founders seemed to be, revolution ok (what they were doing), secession not ok.

Interesting stuff, if quite complex.

(edit) More, on how the actual secession of the southern states fit in :

The primary focus of the declaration is the perceived violation of the Constitution by northern states in not extraditing escaped slaves (as the Constitution required in Article IV Section 2) and actively working to abolish slavery (which they saw as Constitutionally guaranteed and protected). The main thrust of the argument was that since the Constitution, being a contract, had been violated by some parties (the northern abolitionist states), the other parties (the southern slave-holding states) were no longer bound by it.