A lawyer writes in:
One note about the "enemy combatant" controversy, including Jay Carney's new dumb statement confusing "enemy combatant" status with "military commission" trials. Take a look at the Supreme Court's decision in Hamdi v. Rumsfeld (2004), the case that involved a U.S. citizen who was an enemy combatant.
The court plainly ruled in that case that U.S. citizens can be held as enemy combatants. The court's four-justice plurality (which included O'Connor, Breyer, Kennedy, and Rehnquist) was unequivocal on this: "There is no bar to this Nation's holding one of its own citizens as an enemy combatant." And while the U.S. cannot hold an American enemy combatant "indefinitely," it is a "clearly established principle of the law of war" that detention can last as long at the hostilities do.
Now, the Court later held (in Boumediene v. Bush (2008)) that persons deemed to be "enemy combatants" have a right to challenge that designation in court. But that did not change the court's previous holding that U.S. citizens can be enemy combatants. Nothing in the law stops President Obama from determining whether Dzhokhar Tsarnaev an "enemy combatant." It can go ahead and choose to try him in criminal court, rather than a military commission, but that's a completely different question.