Requesting some advice on a question

I’m not sure if this is the right place to put this, but there’s a problem I can tell I’m biased on so I’m requesting some advice from people unlikely to be biased on it.

Legally, circa the time of the actual secessions leading to the creation of the C.S.A, was secession legal? As far as I can tell it was (the Supreme Court may have practical control but they aren’t infallible, and because of the Tenth Amendment), but I can tell I have biased emotions on the subject so I’m checking with people less likely to.