Two months ago a Justice Department lawyer assured U.S. District Judge Anna Diggs Taylor that the National Security Agency's warrantless surveillance of international communications involving people in the United States was perfectly legal. He declined to elaborate, saying "the evidence we need to demonstrate to you that it is lawful cannot be disclosed without that process itself causing grave harm to United States national security."

You have to admire the self-supporting logic of the Bush administration's position: The surveillance is legal if it's necessary to protect national security, and the fact that we're not willing to discuss it shows it's necessary to protect national security. This combination of unilateralism and secrecy is handy for a president but dangerous for a constitutional republic.

As became clear in her recent decision declaring the NSA surveillance illegal and unconstitutional, Judge Taylor did not buy the government's claim that the state secrets privilege, which applies to information that may be detrimental to national security, barred the American Civil Liberties Union from challenging the program in court. But the reason she gave does not bode well for efforts to rein in an executive branch that has declared itself above the law in matters involving terrorism.

Taylor, who had access to classified material that was supposed to convince her of the need to stop the ACLU lawsuit, said all the information she required for a ruling was already a matter of public record, since the administration has confirmed the facts on which the challenge is based while offering a vigorous public defense of the surveillance program's legality. "The court finds defendants' argument that they cannot defend this case without the use of classified information to be disingenuous and without merit," she wrote.

But in a less-noticed part of her ruling, Taylor agreed that the ACLU should not be allowed to pursue its challenge of NSA data mining involving telephone records obtained without a court order. Since the government has not confirmed the existence of that program, she ruled, litigation of the data-mining claims would require disclosure of information covered by the state secrets privilege.

The lesson for the Bush administration is clear: If information about an anti-terrorism program of questionable legality happens to leak out, keep quiet. When it comes to national security, secrecy is its own justification.

That principle is especially troubling in light of President Bush's belief that he has the inherent authority to do whatever he considers necessary to fight terrorism, no matter what Congress or the courts say. Although Taylor's opinion has been criticized for its skimpy Fourth Amendment analysis, it effectively lays bear and emphatically rejects the president's attempted subversion of the checks and balances that distinguish the American executive from a strongman who answers only to his own conscience.

While Taylor failed to adequately support her assertion that the NSA's eavesdropping is "obviously in violation of the Fourth Amendment," her conclusion that the Bush administration is violating the Foreign Intelligence Surveillance Act (FISA), which requires warrants for precisely this type of monitoring, is beyond serious dispute. If the administration is right that Congress lifted FISA's requirements when it authorized the use of military force against the terrorists responsible for 9/11, Congress did so without realizing it.

The authorization "says nothing whatsoever of intelligence or surveillance," Taylor noted, and Congress subsequently amended FISA's warrant requirements in ways that would be entirely redundant if the administration's reading of the authorization were correct. She might have added that the Supreme Court implicitly rejected such a broad interpretation when it ruled that the authorization did not give the president permission to rewrite the laws governing military tribunals.

At bottom, though, Bush does not think he needs permission. Given that attitude, Taylor's declaration that "there are no hereditary kings in America and no powers not created by the Constitution" strikes me not as rhetorical excess but as a much-needed civics lesson for the president.