In not sure if this has been posted, but it looks like even CNN can find precedent for trial of US citizens in military courts.
All of this is from the article above:
Some skeptics might point to Ex Parte Milligan, a civil war era case where the Supreme Court decided that military courts did not have jurisdiction over US citizens if civilian courts were open. Though this may seem damning to Trump’s proposal, Ex Parte Quirin, a 1942 case dealing with Roosevelt’s trial of the German saboteurs, suggests otherwise. In Quirin, the court found that two US citizens could be tried before a military tribunal because they were “in violation of the law of war.”
The court distinguished this case from Milligan by noting that Milligan involved a citizen in Union territory who was conspiring to aid the Confederate forces but had not yet done so, and thus was not associated with the enemy. By contrast, the two citizens in Quirin, “associate[d] themselves with the military arm of an enemy government, and with its aid, guidance, and direction enter[ed] this country bent on hostile acts.” Hence, they were “enemy belligerents within the meaning of the Hague Convention and the law of war.”