In President Bush's speech tonight, he invoked the oath he took as President:

The United States of America has the sovereign authority to use force in assuring its own national security. That duty falls to me, as Commander-in-Chief, by the oath I have sworn, by the oath I will keep.

Pointing to one's oath of office when taking radical action is a time-honored political practice. U.S. Supreme Court Chief Justice John Marshall did the same 200 years ago in Marbury v. Madison, the decision wherein the Court asserted its authority to review the constitutionality of actions by the other two branches of the federal government. (Bush's nominees to the federal bench would largely disagree, but at least for the moment, Marbury is water under the bridge.) Marshall wrote:

From these, and many other selections which might be made, it is apparent, that the framers of the constitution contemplated that instrument as a rule for the government of courts, as well as of the legislature. Why otherwise does it direct the judges to take an oath to support it? This oath certainly applies, in an especial manner, to their conduct in their official character. How immoral to impose it on them, if they were to be used as the instruments, and the knowing instruments, for violating what they swear to support!

For the record, here is the oath of office of the President. Only the last four words of this version are controversial. They are not contained in the official version (which is spelled out in Article II of the Constitution), but Presidents customarily say them:

I do solemnly swear that I will faithfully execute the Office of the President of the United States, and will, to the best of my ability, preserve, protect and defend the Constitution of the United States. So help me God.

Opponents of Bush policies will no doubt note that the oath specifically refers only to defense of the Constitution, and not to defense of friends in foreign countries, or even defense of the "homeland" itself. They are also likely to opine that Bush's domestic policies have in fact offended the Constitution, particularly those pesky amendments sometimes known as the Bill of Rights.