14th Amendment. The United States vs Jefferson Davis was wending its way through to the Supreme Court when Johnson just pardoned everybody. Some judges interpreted it one way and others another - it would have been interesting to see what the Supremes would have made of it...and there's been a LOT of civil rights litigation involving this amendment.
There are several sources that say that Chief Justice Chase, who had been a member of Lincoln's cabinet, was opposed to trying Davis for treason because, under the newly-ratified Fourteenth Amendment, he was already barred from holding public office and, taking that to be a punishment for his actions, trying him for treason would amount to double jeopardy.
There is a famous quote attributed to Chase, to the effect that Davis could never be convicted. I've tried to find the original source, but haven't been able to find it so far; the farthest back I get is Vol. III (1974) of Foote's trilogy (p. 1035, my emphasis):
By that time, prominent Northerners — especially those in the legal profession — had seen the weakness of the government's case against Davis and the handful of Confederates yet being held. One who saw it was the Chief Justice who would rule on their appeal in the event that one was needed, which he doubted. "If you bring these leaders to trial it will condemn the North," Chase had warned his former cabinet colleagues in July, "for by the Constitution secession is not rebellion." As for the rebel chieftain, the authorities would have done better not to apprehend him. "Lincoln wanted Jefferson Davis to escape, and he was right. His capture was a mistake. His trial will be a greater one. We cannot convict him of treason. Secession is settled. Let it stay settled."
Given how often that quote is repeated, without citing a verifiable source,
I'm dubious that Chase ever said that. I haven't seen anything that indicates Chase believed that "secession is not rebellion" at any point, and just two years after he supposedly said that, he wrote the majority opinion in White v. Texas (1869), in which he said (my emphasis):
The Union of the States never was a purely artificial and arbitrary relation. It began among the Colonies, and grew out of common origin, mutual sympathies, kindred principles, similar interests, and geographical relations. It was confirmed and strengthened by the necessities of war, and received definite form, and character, and sanction from the Articles of Confederation. By these the Union was solemnly declared to 'be perpetual.' And when these Articles were found to be inadequate to the exigencies of the country, the Constitution was ordained 'to form a more perfect Union.' It is difficult to convey the idea of indissoluble unity more clearly than by these words. What can be indissoluble if a perpetual Union, made more perfect, is not?
But the perpetuity and indissolubility of the Union, by no means implies the loss of distinct and individual existence, or of the right of self-government by the States. Under the Articles of Confederation each State retained its sovereignty, freedom, and independence, and every power, jurisdiction, and right not expressly delegated to the United States. Under the Constitution, though the powers of the States were much restricted, still, all powers not delegated to the United States, nor prohibited to the States, are reserved to the States respectively, or to the people. And we have already had occasion to remark at this term, that cthe people of each State compose a State, having its own government, and endowed with all the functions essential to separate and independent existence,' and that 'without the States in union, there could be no such political body as the United States.' [12] Not only, therefore, can there be no loss of separate and independent autonomy to the States, through their union under the Constitution, but it may be not unreasonably said that the preservation of the States, and the maintenance of their governments, are as much within the design and care of the Constitution as the preservation of the Union and the maintenance of the National government. The Constitution, in all its provisions, looks to an indestructible Union, composed of indestructible States.
When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration, or revocation, except through revolution, or through consent of the States.
Considered therefore as transactions under the Constitution, the ordinance of secession, adopted by the convention and ratified by a majority of the citizens of Texas, and all the acts of her legislature intended to give effect to that ordinance, were absolutely null. They were utterly without operation in law. The obligations of the State, as a member of the Union, and of every citizen of the State, as a citizen of the United States, remained perfect and unimpaired. It certainly follows that the State did not cease to be a State, nor her citizens to be citizens of the Union. If this were otherwise, the State must have become foreign, and her citizens foreigners. The war must have ceased to be a war for the suppression of rebellion, and must have become a war for conquest and subjugation.
Our conclusion therefore is, that Texas continued to be a State, and a State of the Union, notwithstanding the transactions to which we have referred. And this conclusion, in our judgment, is not in conflict with any act or declaration of any department of the National government, but entirely in accordance with the whole series of such acts and declarations since the first outbreak of the rebellion.
And:
[The Texas Military Board], as we have seen, was organized, not for the defence of the State against a foreign invasion, or for its protection against domestic violence, within the meaning of these words as used in the National Constitution, but for the purpose, under the name of defence, of levying war against the United States. This purpose was, undoubtedly, unlawful, for the acts which it contemplated are, within the express definition of the Constitution, treasonable.
So Chase and the majority of the Supreme Court were on the record, in the most explicit way possible, about the nature of the Confederacy, and (by extension) Davis' actions as head of the Confederate government.
@shanniereb