I found an old post about the possible treason trial for President Jefferson Davis by the US Government. The best and brightest Northern legal minds would not take the case to prosecute Jefferson Davis. They knew there was no chance to make a treason charge stick, or to prove that secession from the Union was illegal:
At any rate, the Northern government prepared to try President Davis for treason while it had him in prison. Mr. Conner has observed that: "The War Department presented its evidence for a treason trial against Davis to a famed jurist, Francis Lieber, for his analysis. Lieber pronounced 'Davis will not be found guilty and we shall stand there completely beaten'." According to Mr. Conner, U.S. Attorney General James Speed appointed a renowned attorney, John J. Clifford, as his chief prosecutor. Clifford, after studying the government's evidence against Davis, withdrew from the case. He said he had 'grave doubts' about it. Not to be undone, Speed then appointed Richard Henry Dana, a prominent maritime lawyer, to the case. Mr. Dana also withdrew. He said basically, that as long as the North had won a military victory over the South, they should just be satisfied with that. In other words - "you won the war, boys, so don't push your luck beyond that."
Mr. Conner tells us that: "In 1866 President Johnson appointed a new U.S. attorney general, Henry Stanburg. But Stanburg wouldn't touch the case either. Thus had spoken the North's best and brightest jurists re the legitimacy of the War of Northern Aggression - even though the Jefferson Davis case offered blinding fame to the prosecutor who could prove that the South had seceded unconstitutionally." None of these bright lights from the North would touch this case with a ten-foot pole. It's not that they were dumb, in fact the reverse is true. These men knew a dead horse when they saw it and were not about to climb aboard and attempt to ride it across the treacherous stream of illegal secession. They knew better. In fact, a Northerner from New York, Charles O'Connor, became the legal counsel for Jeff Davis - without charge. That, plus the celebrity jurists from the North that refused to touch the case, told the federal government that they really had no case against Davis or secession and that Davis was merely being held as a political prisoner.
Author Richard Street, writing in The Civil War back in the 1950s said exactly the same thing. Referring to Jeff Davis, Street wrote: "He was imprisoned after the war, was never brought to trial. The North didn't dare give him a trial, knowing that a trial would establish that secession was not unconstitutional, that there had been no 'rebellion' and that the South had got a raw deal."
Here is the rest of the article.