Oath of enlistment
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I, _____, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice. So help me God." (Title 10, US Code; Act of 5 May 1960 replacing the wording first adopted in 1789, with amendment effective 5 October 1962).
He is correct about army officers swearing to the constitution. However, Trump is most certainly not acting like a tyrant or a dictator and the constitution says he is president.
He does have an oath wherein he swears to support and defend the constitution against all enemies both foreign and domestic.
If he can’t see who IS subverting the constitution right now, he shouldn’t be in a position of authority.