A captured enemy combatant, ie a prisoner of war, is not necessarily an alleged criminal and no allegation of criminality is necessary to justify the detention.
This is why the Gitmo detainees are not necessarily entitled to due process. The Bush administration muddied the waters by creating the category of “enemy combatant” and pretending that it was fundamentally different from “prisoner of war.” It isn’t. This becomes more complicated when the enemy combatant is an American citizen, since being both an enemy combatant and a citizen is most assuredly a crime, namely treason by the strict definition.