While no court specifically ruled on a specific person, the Supreme Court does to this layman appear to differ:
The Prize Cases:
"Insurrection against a government may or may not culminate in an organized rebellion, but a civil war always begins by insurrection against the lawful authority of the Government. A civil war is never solemnly declared; it becomes such by its accidents — the number, power, and organization of the persons who originate and carry it on. When the party in rebellion occupy and hold in a hostile manner a certain portion of territory, have declared their independence, have cast off their allegiance, have organized armies, have commenced hostilities against their former sovereign, the world acknowledges them as belligerents, and the contest a war. They claim to be in arms to establish their liberty and independence, in order to become a sovereign State,
while the sovereign party treats them as insurgents and rebels who owe allegiance, and who should be punished with death for their treason.
"The laws of war, as established among nations, have their foundation in reason, and all tend to mitigate the cruelties and misery produced by the scourge of war. Hence the parties to a civil war usually concede to each other belligerent rights. They exchange prisoners, and adopt the other courtesies and rules common to public or national wars.
"A civil war," says Vattel, breaks the bands of society and government, or at least suspends their force and effect; it produces in the nation two independent parties, who consider each other as enemies and acknowledge no common judge. Those two parties, therefore, must necessarily be considered as constituting, at least for a time, two separate bodies, two distinct societies. Having no common superior to judge between them, they stand in precisely the same predicament as two nations who engage in a contest and have recourse to arms.
"This being the case, it is very evident that the common laws of war — those maxims of humanity, moderation, and honor — ought to be observed by both parties in every civil war. Should the sovereign conceive he has a right to hang up his prisoners as rebels, the opposite party will make reprisals, &c., & c.; the war will become cruel, horrible, and every day more destructive to the nation.
"As a civil war is never publicly proclaimed, eo nomine, against insurgents, its actual existence is a fact in our domestic history which the Court is bound to notice and to know.
"The true test of its existence, as found in the writings of the sages of the common law, may be thus summarily stated:
"When the regular course of justice is interrupted by revolt, rebellion, or insurrection, so that the Courts of Justice cannot be kept open, civil war exists, and hostilities may be prosecuted on the same footing as if those opposing the Government were foreign enemies invading the land." [67 US 635, 666-668]
"This greatest of civil wars was not gradually developed by popular commotion, tumultuous assemblies, or local unorganized insurrections. However long may have been its previous conception, it nevertheless sprung forth suddenly from the parent brain, a Minerva in the full panoply of war. The President was bound to meet it in the shape it presented itself, without waiting for Congress to baptize it with a name; and no name given to it by him or them could change the fact.
"It is not the less a civil war, with belligerent parties in hostile array, because it may be called an "insurrection" by one side, and
the insurgents be considered as rebels or traitors. It is not necessary that the independence of the revolted province or State be acknowledged in order to constitute it a party belligerent in a war according to the law of nations. Foreign nations acknowledge it as war by a declaration of neutrality. The condition of neutrality cannot exist unless there be two belligerent parties." [67 US 635, 668-669]
White v. Hart:
"
The doctrine of secession is a doctrine of treason, and practical secession is practical treason, seeking to give itself triumph by revolutionary violence. The late rebellion was without any element of right or sanction of law. The duration and magnitude of the war did not change its character. In some respects it was not unlike the insurrection of a county or other municipal subdivision of territory against the State to which it belongs. In such cases the State has inherently the right to use all the means necessary to put down the resistance to its authority, and restore peace, order, and obedience to law. If need be, it has the right also to call on the government of the Union for the requisite aid to that end. Whatever precautionary or penal measures the State may take when the insurrection is suppressed, the proposition would be a strange one to maintain, that while it lasted the county was not a part of the State, and hence was absolved from the duties, liabilities, and restrictions which would have been incumbent upon it if it had remained in its normal condition and relations. The power exercised in putting down the late rebellion is given expressly by the Constitution to Congress. That body made the laws and the President executed them. The granted power carried with it not only the right to use the requisite means, but it reached further and carried with it also the authority to guard against the renewal of the conflict, and to remedy the evils arising from it in so far as that could be effected by appropriate legislation. At no time were the rebellious States out of the pale of the Union. Their rights under the Constitution were suspended, but not destroyed. Their constitutional duties and obligations were unaffected and remained the same." [80 US 646, 650-651]
In his opinion in US v. Burr, Chief Justice John Marshall wrote, "That part of his deposition which bears upon this charge is the plan disclosed by the prisoner for seizing upon New Orleans, and revolutionizing the Western states. That this plan, if consummated by overt acts, would amount to treason, no man will controvert." [25 Fed. Cas. 13] He seems to be saying, in my most un-expert opinion, that instance of internal rebellion would be treason.
James G. Randall considered the Treason Law in a chapter of his book, Constitutional Problems Under Lincoln: "By judicial interpretation a fairly definite body of principles came gradually to be built up around the general subject of treason. 'Constructive treason' was eliminated. There must be an actual levying of war. A mere plotting, gathering of arms, or assemblage of men is not treason. The overt act of treason must be proved before collateral testimony can be admitted tending to connect a particular person with such treasonable activity.
"
The 'levying war' includes not only formal or declared war, but also any combination to interfere by force with the execution of any law of the United States. An insurrection to obstruct the execution of an act of Congress is treason, since it amounts to levying war. Enlisting, or procuring enlistment in the enemy's service, is treason; though persuading men to enlist is not, unless consummated by actual enlistment. The mere uttering of words bearing a treasonable import does not constitute the crime. Mere expressions of sympathy with the enemy, although sufficient to justify the suspicion that one is at heart a traitor, are not sufficient to warrant conviction for treason." [pp. 75-76]
The US Congress in 1862 also appears to differ. For example, the official title of the Second Confiscation Act is, "
An Act to Suppress Insurrection; to punish Treason and Rebellion, to seize and confiscate the Property of Rebels, and for other purposes."
In the case of US v. Jefferson Davis, Chief Justice Salmon Chase said that the 14th Amendment had already punished Jefferson Davis for treason, and therefore he couldn't be tried for treason because it would be double jeopardy. He appears to differ.
I don't know of any authorities who have said treason does not apply to an internal rebellion.