Whoa, you've presented a very interesting scenario. Who do you think it would be that would carry out the feat you have presented?
If you would rather keep it private, please FM me, I'd really like to know.
In a perfect world, it would be the 3rd US Infantry Regiment from Ft. Myer, aka ‘The Old Guard’, that would be highly appropriate if for no other reason than ceremonial.
In reality it could be any special unit that we may or may never have heard of before, why? Because I refuse to believe that there have not been contingency plans on the shelf in all branches of our military to deal with the remote possibility that a foreign flag agent might somehow gain control of the mechanisms of the Executive Branch, specifically the authority and command functions of the POTUS.
In such a situation, it would be my expectation that a specific unit, most likely assembled from special operation types of all four military branches, none of them knowing each other, but all of them having specific detailed orders not unlike the sealed launch codes used by SAC during the Cold War. In this case the ‘launch code’ would be for the apprehension and arrest of one or more clearly identified individuals acting in behalf of America’s adversaries. Such a unit and the orders issued to the members of that unit are most likely not documented in order to preserve the operational integrity of the unit.
As to who gives the activation order to such a unit?
That is the $64,000 question.