I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.(Article II, Section 1). The words "enemy", whether domestic or foreign, do not appear in the oath.
Job one then, the only thing a president swears or affirms he or she will do, is "preserve, protect and defend the Constitution of the United States".
It would seem then that the enemy is anyone who would damage the US Constitution. An "enemy combatant", by extension, would be anyone who would damage the US Constitution by combat.
The federal court in Washington is giving the new Obama administration an opportunity to define the term.
The Bush regime defined it as anyone the president says is an "enemy combatant" whether they did combat against the US Constitution or not. Not so clear.
What is clear is that a person can obviously be an enemy or an enemy combatant, depending on whether or not combat is involved.
I have a hunch that the new Obama administration will say "enemy combatant" is any non-citizen who seeks to harm the US Constitution by combat.
There is another part of constitutional law that deals with citizens who would take up arms against the US Constitution. It is called treason.
Treason is mentioned in Art. I, Sec. 6, Art. II, Sec. 4, and is defined as:
Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.(Article III, Section 3). Since Article III is the one that deals with the Judicial Branch, the courts, it would seem that such behaviour is for the courts to handle, not for military commissions or tribunals to mumbo jumbo through.
The Congress shall have Power to declare the Punishment of Treason, but no Attainder of Treason shall work Corruption of Blood, or Forfeiture except during the Life of the Person attainted
The military is for military matters, and the courts are for judicial matters.