Once again, you assume that secession was not 'legal.' Feel free to read the Tenth Amendment, and then quote the constitutional clause that 'delegates or prohibits' secession. In fact, it was the federal government that refused to settle the issue "peaceably:" barring any constitutional prohibition of secession, the federal troops who remained at Fort Sumter were nothing but armed, foreign trespassers - who were eventually evicted without loss of life...
;>)
Now, I don't know what you might mean by the term "secession," but I know what the southern politicians trying to protect slavery meant. They intended to divest the government of the United States of its powers in the southern states. They really meant that.
The constitution does not provide for any right of "secession." (They just made the whole thing up.) The only constitutional way to divest the powers of the government of the United States is to amend the constitution pursuant to Article V. The southern states did not wish to take that route and they didn't even want to litigate the legitimacy of their position. Instead, they just told the government of the United States to take a hike. They didn't care about the wishes or desires of the people of the United States, or the governments of the other states, or the government of the United States.
At the time, the southern politicians involved in all of this were desperate. They genuinely believed that the preservation of slavery was vital to the preservation of the southern culture. So they took a gamble. And they lost. And now we know that the preservation of slavery was not vital to the preservation of southern culture.