As a tax person, I am experienced at reading and construing statutes. So, despite my lack of legal background in the precise area of the military commissions legislation, I thought I would give it a careful read. Having done so, I must say that I am baffled and suspicious.
The stated purpose of the legislation is to "establish procedures governing the use of military commissions to try alien unlawful enemy combatants engaged in hostilities against the United States for violations of the law of war and other offenses triable by military commission." Section 948b(a).
Towards this end, the legislation contains two separate definitions of particular interest. One is "unlawful enemy combatant," defined in either of two ways. The first is as "a person who has engaged in hostilities or who has purposefully and materially supported hostilities against the United States or its co-belligerents who is not a lawful enemy combatant" (i.e., a member of regular armed forces somewhere other than the Taliban or al Qaeda). Section 948a(1)(A)(i). Many have noted how broad this language is. E.g., Vice President Cheney characterizes various forms of political dissent in terms that don't fall far short of this.
An "unlawful enemy combatant" is also defined as anyone who "has been determined to be an unlawful enemy combatant by a Combatant Status Review Tribunal or another competent tribunal established under the authority of the President or the Secretary of Defense." Section 948a(1)(A)(ii). Many have noted that this appears to be standardless. You and I are unlawful enemy combatants if Bush or Rumsfeld establishes a tribunal that so finds under whatever standards they happen to prescribe.
But here's where the plot thickens. Again, the legislation serves to to "establish procedures governing the use of military commissions to try ALIEN unlawful enemy combatants" (section 948b(a); emphasis added).
To meet that definition, you must also be an "alien," which is separately defined in section 948a(3) as "a person who is not a citizen of the United States."
By virtue of section 948c, "[a]ny alien unlawful enemy combatant is subject to trial by military commission under this chapter."
Thus, there is nothing in the provision that gives separate legal significance to the term "illegal enemy combatant" without the word "alien" in the front.
So the question is: Why does the legislation define a term, "unlawful enemy combatant," that has no legal significance under it whatsoever if not preceded by the term "alien"? Sloppy drafting is one possibility. Skilled tax statutory drafters, at least with the time to check their work, would never leave a freefloating term like that.
But strange times breed mistrust. Is there a reason for gratuitously defining "unlawful enemy combatant" so that it unambiguously can be met by an American citizen, even absent operative provisions in this legislation itself that turn on meeting the definition?
An additional ambiguity here is that the provision I read is a subchapter, and the definitions are stated to apply for purposes of the entire chapter. What are the other subchapters? This may be knowable, but I don't happen to know it.
The upshot: this legislation provides for the use of military commissions solely against non-citizens. But it apparently gratuitously defines "unlawful enemy combatant" in a way that would permit the Administration to determine that an American citizen is such an individual. Indeed, it's purely discretionary with the President and the Secretary of Defense.
It is easy to conclude that the Administration will treat this determination as legally relevant to what it can do to American citizens, even though on the face of the legislation it can't use it to try them before military commissions.
But why bother to try them anyway, especially if you believe that you are empowered to detain and torture illegal enemy combatants indefinitely without any requirement that they be subject to a specified set of trial procedures?
I therefore conclude as follows: The legislation is not directly relevant to the question of what the Administration can do to American citizens. But it provides a statutory basis for describing them as "unlawful enemy combatants," which I would expect the Administration to treat as having further independent legal significance.
UPDATE: Marty Lederman reminds me that the law of war is conventionally interpreted (including in the Supreme Court's Hamdi decision) to permit detention of enemy combatants for the duration of the war for purposes of incapacitation.
The statute therefore arguably provides quite important if indirect statutory support for Bush's claim that he has absolute power of arrest and detention over all citizens as well as non-citizens.