Col. Robert E. Lee, US Army, resigned his commission, which was accepted April 25, 1861.
Robert E. Lee was appointed general in the Militia of the State of Virginia, commanding the State's military and naval forces April 23, 1865.
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Was holding a State troops/militia commission and a US Army commission simultaneously a breach of oath? I doubt it.
April 27, the State of Virginia organizes a provisional army for defense..., commanded by Maj. Gen. Lee of the State of Virginia.
April 17, 1861; Virginia convention votes secession from the USA, pending a referendum vote (held and ratified, May 23).
April 25, 1861 the State of Virginia entered into a temporary convention with the Confederate States of America...
May 23, 1861, VA ratifies ordinance of secession after referendum vote in the affirmative.
May 24 US troops enter Virginia... at Arlington If I recall correctly...
June 19, 1861: the State of Virginia ratifies the Constitution of the CSA, and declares itself bound to it...
July 13, 1861: The United States Government Act declares a large part of the State of Virginia in rebellion... August 16, 1861 Pres. Lincoln by proclaims the people of Virginia, east of the Alleghenies, (including Gen. Lee) in rebellion against the USA...
Lee was commissioned as a General in the CS Army, in August, 1861 to rank from June 21, 1861. In early 1865 he was appointed General-in-chief of the CS Army...
Parole of Gen. Lee accepted by the US Army at Appomattox Court house, VA, April 9, 1865:
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And general Lee was never "properly exchanged" by POW cartel to allow him to serve the CSA in any military capacity before the close of the war...
Oath of allegiance to the United States, signed Oct. 2, 1865:
View attachment 416632
In 1975, Lee's full rights of citizenship were posthumously restored by a joint congressional resolution effective June 13, 1865.
During his lifetime he was neither tried or convicted of treason. (If anyone is aware of a Confederate soldier or sailor convicted of treason, I would be interested in knowing).
The Constitution of the US defines treason as:
"Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court.
The Congress shall have power to declare the punishment of treason, but no attainder of treason shall work corruption of blood, or forfeiture except during the life of the person attainted."
There appears to be many yet who would like to act as witnesses for some sort of prosecution.