Mark is absolutely correct that there may be good reasons not to try certain captured U.S. citizen combatants for treason. But the Constitution explicitly provides for this by allowing Congress to suspend habeas. Indeed, it is notable that the Constitution makes specific reference to the suspension of habeas in Article I. Indeed, under the Constitution, habeas is alive and well so long as the courts remain open, even in the case of invasion.
If the President believes it is necessary to detain U.S. citizen enemy combatants indefinitely to interrogate them, then the exeuctive should request a suspension of habeas for such purposes. There is nothing in the Constitution precluding Congress from so limiting the suspension, and it has done so in the past, so habeas could remain operable for other cases. Congress need not declare martial law to remove habeas protections for enemy combatants. The question upon which Mark and I disagree (and, incidentally, part of the grounds upon which Justices Scalia and Thomas differ) is whether the President may order such indefinite detention absent such Congressional approval. When it comes to U.S. citizens who are detained on U.S. soil, this is something Congress must approve. (Again, however, I would note that Mark and I agreee that the courts are without jurisdiction to hear such claims from those who are not within the sovereign territory of the U.S.)