Interesting. Here's what a prominent legal mind of the civil war era had to say on that subject. He was also a leader in the abolitionist movement, BTW, and spent many years prior to the war offering legal defense free of charge to fugitive slaves....
"The Constitution says:
"Treason against the United States shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort."
This is the only definition of treason given by the Constitution, and it is to be interpreted, like all other criminal laws, in the sense most favorable to liberty and justice. Consequently the treason here spoken of, must be held to be treason in fact, and not merely something that may have been falsely called by that name.
To determine, then, what is treason in fact, we are not to look to the codes of Kings, and Czars, and Kaisers, who maintain their power by force and fraud; who contemptuously call mankind their "subjects;" who claim to have a special license from heaven to rule on earth; who teach that it is a religious duty of mankind to obey them; who bribe a servile and corrupt priest-hood to impress these ideas upon the ignorant and superstitious; who spurn the idea that their authority is derived from, or dependent at all upon, the consent of their people; and who attempt to defame, by the false epithet of traitors, all who assert their own rights, and the rights of their fellow men, against such usurpations.
Instead of regarding this false and calumnious meaning of the word treason, we are to look at its true and legitimate meaning in our mother tongue; at its use in common life; and at what would necessarily be its true meaning in any other contracts, or articles of association, which men might voluntarily enter into with each other.
The true and legitimate meaning of the word treason, then, necessarily implies treachery, deceit, breach of faith. Without these, there can be no treason. A traitor is a betrayer --- one who practices injury, while professing friendship. Benedict Arnold was a traitor, solely because, while professing friendship for the American cause, he attempted to injure it. An open enemy, however criminal in other respects, is no traitor.
Neither does a man, who has once been my friend, become a traitor by becoming an enemy, if before doing me an injury, he gives me fair warning that he has become an enemy; and if he makes no unfair use of any advantage which my confidence, in the time of our friendship, had placed in his power.
For example, our fathers --- even if we were to admit them to have been wrong in other respects --- certainly were not traitors in fact, after the fourth of July, 1776; since on that day they gave notice to the King of Great Britain that they repudiated his authority, and should wage war against him. And they made no unfair use of any advantages which his confidence had previously placed in their power.
It cannot be denied that, in the late war, the Southern people proved themselves to be open and avowed enemies, and not treacherous friends. It cannot be denied that they gave us fair warning that they would no longer be our political associates, but would, if need were, fight for a separation. It cannot be alleged that they made any unfair use of advantages which our confidence, in the time of our friendship, had placed in their power. Therefore they were not traitors in fact: and consequently not traitors within the meaning of the Constitution.
Furthermore, men are not traitors in fact, who take up arms against the government, without having disavowed allegiance to it, provided they do it, either to resist the usurpations of the government, or to resist what they sincerely believe to be such usurpations.
It is a maxim of law that there can be no crime without a criminal intent. And this maxim is as applicable to treason as to any other crime. For example, our fathers were not traitors in fact, for resisting the British Crown, before the fourth of July, 1776 --- that is, before they had thrown off allegiance to him --- provided they honestly believed that they were simply defending their rights against his usurpations. Even if they were mistaken in their law, that mistake, if an innocent one, could not make them traitors in fact.
For the same reason, the Southern people, if they sincerely believed --- as it has been extensively, if not generally, conceded, at the North, that they did --- in the so-called constitutional theory of "State Rights," did not become traitors in fact, by acting upon it; and consequently not traitors within the meaning of the Constitution." - Lysander Spooner, 1870
The obvious rejoinder is that, by seceding from "the United States" and then levying war against them, in plain fact they committed treason. From this perspective, it does not matter even if the North started the war. Any armed resistance would be "levying war" and thus could be defined as treasonous.
And I say that with half of my ancestors on each side of that conflict.
Of course, clearly most of those in the South did not think of their actions as treasonous, simply because they no longer considered themselves as citizens of the United States. But think of this...if a whole bunch of Islamic types today moved to a sparsely populated state and tried to secede, and used violence to defend their secession - would it be treason? If yes, then it was treasonous then. If not, then it wasn't.