>and come to think of it, the Military Tribunals for American citizens at Gitmo sounds preposterous.
There are certainly challenges. The question hinges upon the availability of our civilian court system, which may of course be compromised. There is also the possibility that US citizens in some cases may be determined to be enemy combatants, although that is open to argument as well.
Ex parte Milligan, 71 U.S. 2 (1866)
A citizen not connected with the military service and a resident in a State where the courts are open and in the proper exercise or their jurisdiction cannot, even when the privilege of the writ of habeas corpus is suspended, be tried, convicted, or sentenced otherwise than by the ordinary courts of law.
https://supreme.justia.com/cases/federal/us/71/2/
Ex Parte Milligan, (1866), case in which the U.S. Supreme Court ruled that the federal government could not establish military courts to try civilians except where civil courts were no longer functioning in an actual theatre of war.
https://www.britannica.com/event/Ex-Parte-Milligan