Unlawful enemy combatant.
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Unlawful = Not a soldier of a foreign power who has sworn loyalty to his nation and to uphold his nation's laws and obey his commanders.
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Enemy = intends to do harm to the USA, plans to do harm to the USA, conspires to do harm to the USA.
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Combatant = engages in combat activities including ordering them, supplying weapons, resources etc.
So, what if a citizen of the USA engages in combat against the USA? They are almost certainly all 3. If they have sworn loyalty to the constitution than any betrayal of it is clearly unlawful.
This means that any blackhat in public office is fucked. If they just embezzled some public funds to buy a yacht, that would be OK. Simple criminal. But banding together with others some of whom kill other public officials makes you accessory to murder and now you are part of the cult of terrorism.