One [FBI] agent, frustrated at encountering the "wall" [separating intelligence officials from criminal investigators], wrote to headquarters [on Aug. 29, 2001]: "Someday someone will die and—wall or not—the public will not understand why we were not more effective and throwing every resource we had at certain 'problems.' The biggest threat to us now, UBL [Osama bin Laden], is getting the most 'protection.' "—Opinion of Foreign Intelligence Surveillance Court of Review, November 18, 2002

Complexity can confound clear thinking and facilitate false alarms. Such has been the case with the claims that the above-quoted ruling by the special three-judge review court eviscerates our liberty and privacy rights. The powerfully reasoned decision held that the government may use the Foreign Intelligence Surveillance Act to wiretap and search suspected agents of foreign terrorist groups and governments even if its primary goal is criminal prosecution, as distinguished from pure intelligence-gathering. It also dismantled the legal "wall" that has impeded intelligence officials from working with criminal investigators—a wall that may have made it easier for the September 11 hijackers to do their evil work.

But the complexity of issues involving FISA has enabled critics to cry wolf in a most misleading fashion, while some of the same folks who have deplored the failure of intelligence and law enforcement officials to work together now lament a ruling that will help them do just that.

The "misguided" review court, yelped The New York Times, "handed the government broad new authority ... to wiretap phone calls, intercept mail, and spy on Internet use of ordinary Americans." This ruling, asserted the American Civil Liberties Union, "will affect every American's privacy rights" and "suspend the ordinary requirements of the Fourth Amendment."

Well. It was a big win for Attorney General John D. Ashcroft, whose habit of pushing his powers to dubious extremes does scare many of us. But the reports of liberty's death are greatly exaggerated. Let's explore 1) the previously obscure realm of FISA, which has become a critical tool in our government's efforts to stop terrorists before they kill us; 2) how the now-defunct "wall" has hindered those efforts; and 3) the modest impact of this decision on the more than 280 million Americans who are not foreign agents.

FISA and the special courts. FISA was enacted in 1978, initially for electronic surveillance, and later amended to cover physical searches, too. It required presidents—who had previously claimed unilateral power to wiretap or search suspected foreign agents—to obtain judicial warrants. At the same time, it relaxed the usual Fourth Amendment rule requiring "probable cause" to suspect criminal activities before approving wiretaps or searches, in light of the difficulty of producing such evidence in investigations of suspected foreign agents, who are trained to avoid suspicious activities. FISA requires somewhat less evidence—probable cause to suspect that the target is a foreign agent—while authorizing more-intrusive surveillance, for longer periods, under greater secrecy.

The statute created two special federal courts that operate in extraordinary (and to some extent unwarranted) secrecy. The so-called FISA court, recently expanded from seven to 11 judges, considers applications for warrants. The review court—federal appellate Judges Laurence H. Silberman of Washington, Edward Leavy of Portland, Ore., and Ralph B. Guy Jr. of Ann Arbor, Mich., all Reagan appointees—hears any government appeals from the FISA court. Both are staffed by federal judges from around the country designated by Chief Justice William H. Rehnquist to work part-time on FISA matters. The review court's November 18 decision was its first ever: The FISA court has approved almost all of the Justice Department's more than 14,000 warrant applications since 1978. This has led critics to dismiss the FISA court as a meaningless rubber stamp. Recent revelations suggest otherwise, and the government's batting average may instead reflect a policy of seeking informal guidance first and filing formal applications only when approval seems assured.

The FISA court showed its teeth earlier this year in its much-publicized 7-0 decision that rejected Ashcroft's proposal to tear down the "wall," which is shorthand for a tangle of FISA court rules designed to prevent the government from seeking FISA warrants primarily for use in criminal prosecutions. The USA-PATRIOT Act explicitly discarded some of these rules. But the FISA court adhered to others that barred prosecutors from advising intelligence officials on FISA matters and required that official "chaperones" attend meetings to ensure compliance. The review court reversed this decision. It also rejected the implication in the FISA court's opinion—which was internally inconsistent and thus incoherent on this point—that the government may not use FISA when its primary purpose is to prosecute terrorist conspiracies, espionage, or similar crimes.

How the "wall" protected terrorists. The FISA court was right to bar the use of FISA to seek evidence for ordinary prosecutions. (It's undisputed that any evidence of crime that turns up as an incidental result of a FISA search or wiretap can be so used.) But its notion that FISA could not be used to facilitate even prosecutions for terrorist conspiracies or espionage turned FISA's stated goal of "protect[ing] against" such dangers on its head. In the review court's words, "Arresting and prosecuting terrorist agents ... or spies may well be the best technique to prevent ... their terrorist or espionage activity." The "wall" not only made investigators wary of talking to one another. It also spurred unnatural contortions to eschew the natural impulse to use FISA to protect against terrorists or spies by locking them up.

"Indeed," the review court noted, "it was suggested that the FISA court requirements ... may well have contributed, whether correctly understood or not, to the FBI missing opportunities to anticipate the September 11, 2001, attacks.... An FBI agent recently testified [in congressional hearings] that efforts to conduct a criminal investigation of two of the alleged hijackers were blocked by senior FBI officials—understandably concerned about prior FISA court criticism [for undisclosed or concealed breaches in the 'wall']—who interpreted that court's decisions as precluding a criminal investigator's role." Citing the FBI agent's bitter complaint that "someday someone will die"—because of the refusal of FBI headquarters to launch an aggressive search for Khalid Almihdhar, whom the CIA had identified as a bin Laden follower—the review court noted that the official response had been that "those were the rules, and [FBI headquarters] does not make them up." Thirteen days later, Almihdhar helped crash an airliner into the Pentagon.

Why did the FISA court create such strange rules? They are rooted in decisions by other federal courts that the Fourth Amendment bars seeking a FISA warrant primarily for the purpose of prosecution. Those decisions in turn cite Supreme Court precedents arguably establishing a general rule that any searches and wiretaps aimed primarily at prosecution must be authorized in advance by warrants based—unlike some FISA warrants—on "probable cause" to suspect criminal activity. But that rule was not designed to deal with the terrorists who now threaten us with mass murders dwarfing the harm done by ordinary criminals. And almost all rules have exceptions.

Why the wall's fall leaves liberty and privacy alive and well. The review court's decision does not lighten by one iota the government's burden of showing—before it can use FISA to wiretap you, me, the neighborhood drug dealer, or the Muslim family down the street—that the target is an agent of a foreign terrorist group or government. And the definition of a foreign "agent" includes U.S. citizens only when there is evidence implicating them in conduct that crosses the line of criminality or comes very close to it: "sabotage or international terrorism," which are crimes; activities "in preparation therefor"; or "clandestine intelligence-gathering activities [that] involve or may involve a [criminal] violation."

It's true that this decision clears the way for more wiretaps and searches, because the government can now use FISA when its primary purpose is prosecution. But critics are quite wrong to suggest that this opens the way for FISA surveillance to seek evidence of ordinary crimes. Few criminals or suspects are foreign agents. And the review court rejected the administration's argument that the USA-PATRIOT Act authorized use of FISA to seek to implicate foreign agents in ordinary crimes.

"FISA as amended is constitutional," the review court ruled, "because the surveillances it authorizes are reasonable."

Correction: My last column stated incorrectly that federal appellate Judge Samuel A. Alito Jr. had been a law clerk for Justice Antonin Scalia. I regret the error.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.