During the Second World War, six German saboteurs were landed by submarine, on the East coat. They were captured and tried by Military Tribunal. One was a US Citizen.
The Supreme Court ruled that they were tried properly, and they were executed.
If the Supreme Court found that to be constitutional, it is fair to state that the constitution provides for Military Commissions or Tribunals.
After the war, the Chief Justice presided over a Military Tribunal in Germany where numerous individuals were tried, convicted and executed.
I am certain that if the Chief Justice headed a Military Tribunal, it wasn't unconstitutional.
Well, I don't have the same confidence in their ability to always get the Constitution right, but like I said, it's not on the same level with the explicit constitutional provisions for civilian courts and the whole system of checks and balances. As such, it shouldn't be taken for granted, and even if we assume that the supreme court got it right in that particular case, it needs to be examined in the full context of that case, so as to avoid drawing too many unwarranted conclusions from it.