No record of Lee's oath as colonel exists, but he definitely pee'd all over this one:
"I, Robert E. Lee, appointed a Lieutenant Colonel of the Second Regt. of Cavalry in the Army of the United States, do solemnly swear, or affirm, that I will bear true allegiance to the United States of America, and that I will serve them honestly and faithfully against all their enemies or opposers whatsoever; and observe and obey the orders of the President of the United States, and the orders of the Officers appointed over me, according to the Rules and Articles for the government of the Armies of the United States."
Some of the neo-rebs will doubtless be along to muddy the waters, but it an historical fact that Lee took up arms for the rebel government while still under oath to the United States.
"According to paragraph 24 of the Regulations of the United States Army, 'No officer will be considered out of service on the tender of his resignation, until it shall have been duly accepted by the proper authority.' Paragraph 28 states that "in time of war, or with an army in the field, resignations shall take effect within thirty days of from the order of acceptance.' But Lee disregarded these restrictions. Between the posting of his resignation on April 20 and its acceptance on April 25, Lee drew his sword with remarkable alacrity." ("Lee Considered" by Alan Nolan, p. 39)
Lee met with rebel officials on April 20, well before his resignation was accepted.
Walt