Just happened to catch a video on the Shelby Foote site with a lecture from Allen Guelzo on whether Robert E. Lee was guilty of treason.  Guelzo says yes but at the same time although Lee was indicted by a jury in Virgina he was never brought to trail and therefore never formally found guilty of treason.  A document Lee agreed to with Grant at his surrender seemed to absolve him and other Confederate officials of being accused of treason.  The legality of this document was controversial at the time.  Lee was viciously pursued by a Judge Underwood in Virginia.  He would have been tried in Virginia and it seems unlikely that a Virginia jury would have found him guilty.  Supreme Court Salmon Chase wanted no part of trying Lee for treason.  Ambiguity in the Constitution over the meaning of citizenship was Lee's main defense.  The Constitution talks about US citizenship and state citizenship.  Lee said he was acting in the capacity of his state citizenship.