No one can. You are thinking of Article 138 of the UCMJ, maybe? Unfortunately it can not be used on the President, as, in his person as Commander in Chief, he has no superior officer and a superior officer must be available to arbitrate if redress is not obtained.
Are there emergency actions that are allowed when lives (lots of them) are in danger? For instance, if Obama had done what Nidal Hasan had done, could anybody have stopped him - whether in the military or civilians? Would everybody just have to let him kill as many people as he wanted until Congress could be gathered together, vote to impeach him, have the trial, and convict him? Or maybe declare him insane (however that is done)?
And just to clarify: have you been through officers’ training? If so, are you saying that these scenarios are not discussed, as was claimed by the person in the quote I gave?
Thanks for any input.