Terror in the skies: and some down-to-earth lessons.

A BREATHTAKING jihadist plot to blow as many as ten U.S. airliners out of the trans-Atlantic sky after take-off from London's Heathrow Airport could well have reprised the horror of 9/11. Still, however thankful all of us must be about the two dozen arrests that have taken place in England and Pakistan since August 10, the preemption of this scheme will prove very discomfiting for liberals and civil-libertarian extremists who have barnstormed for five years against all sensible counterterrorism measures.

First and foremost, the successful thwarting of this plot demonstrates again that there is no substitute for spies. While the investigation was a model of international cooperation and brought to bear several sophisticated surveillance techniques, it appears to have been triggered by an informant's tip--after which authorities managed to infiltrate the terror plot with an undercover agent. While perhaps not privy to the highest echelon of the vast conspiracy--which may have involved more than four dozen operatives--the agent gave investigators access to key details over many months.

Spies--known in the trade as "human intelligence"--have been a much-neglected element of the American counterterrorism debate. We have focused, instead, on such legal esoterica as procedures for wiretapping and access to business records. This reflects an oddity in the civil-libertarian worldview, because there is no investigative tactic more intrusive, more violative of privacy, than government's deployment of undercover agents and confidential informants. Yet no prior judicial permission is necessary before they may be sent into a house of worship (or any common area), or invited into the private home of a suspect, or tasked to record their own conversations with investigative targets--even though they are plainly misrepresenting themselves. The lack of complaint over this from activists set on auto-thrombosis over such comparative trifles as government access to library records may be calculated. After all, the tactic is not merely incontestably legal; it is dramatically more effective than any other in preventing terrorism and violent crime.

Most of the prominent national-security cases of the 1990s--when terrorism was regarded as a crime to be prosecuted rather than an act of war to be preempted--involved completed attacks and the consequent loss of life. Only once did law enforcement actually manage to disrupt a plot, amass compelling evidence against all the participants, and convict them. That was the so-called "Day of Terror" conspiracy in 1993 to bomb New York City landmarks. As it happens, I led that investigation, but what was different about it had nothing to do with the prosecutors, agents, or even the terrorists. The difference was human intelligence: an informant who provided a window into the plot as it was unfolding. This saved thousands of lives.

Despite the obvious importance of such measures, the capacity of the U.S. intelligence community to plant moles in the terror networks most threatening to Americans was drastically degraded after the Cold War, particularly during the Clinton years. Thus, real public attention to this topic would severely damage the credentials of many a grandstanding critic of "intelligence failure." Dots, after all, must be collected before they can be connected. As such critics, naturally, are celebrated allies of privacy activists, spying is a topic better left unexplored.

Even more inconvenient for Bush-bashers is the role of coercive interrogation tactics. It now seems clear that the audacious British plan was stopped in the very late stages--and suspects were abruptly rounded up before they could either start the bombing or flee--because Pakistani authorities arrested top suspect Rashid Rauf and subjected him to questioning that, to put it mildly, did not involve Miranda warnings or other enlightened Western practices.

Did the coercion of Rauf amount to torture? Even if not, is it morally acceptable to take and act on information generated by coercive measures--such as, say, sleep deprivation--in order to save thousands of innocent lives? These should be excruciating questions, given the high-minded rhetoric that drove last year's passage of the McCain amendment barring "cruel, inhuman, or degrading treatment" of detainees.

Obviously, torture is a third rail. Although it is easy to imagine worse evils, it is also easy to understand why civilized people would insist on an absolute ban (even as many of them privately hope some Jack Bauer type will ignore the ban when extreme circumstances arise). But does it really make sense, in a threat environment dominated by terrorists bent on mass murder, to prohibit any type of "degrading" treatment--whatever that hopelessly vague term may mean--if resorting to it could prevent civilian jumbo jets from being exploded in flight?

Like interrogation, detention has been a counterterror hot button since 9/11. With the smashing of the airplane plot, it has also become focal to an argument about whether British or American law provides the superior counterterrorism arsenal. It would be illusory, though, to pronounce either system "better" overall: Each has features the other should consider adopting.

England has abundant terrorism experience, including its struggles with the IRA. Consequently, it is more hard-wired toward intelligence and the prevention of attacks than the U.S., which, until recently, had little need to worry about preventing catastrophic strikes and thus emphasized investigation and prosecution. Detention practice is one of several areas in which this difference is palpable. In England, preventive detention is unabashedly legal and thought particularly appropriate for combating terrorism. Suspects can be arrested and held for up to four weeks without being charged. The internal debate in the U.K. involves not whether preventive detention is warranted, but whether 28 days is enough.

In the U.S., preventive detention absent formal charges is generally impermissible. Thus the government, confronted by the present-day reality that there is often insufficient usable evidence to charge potential terrorists who can't responsibly be allowed to walk the streets, has had to find roundabout ways to sideline dangerous suspects. This has resulted in the controversial use of material-witness arrest warrants (which are really designed to preserve testimony, not prevent violence or build cases against those thus detained) and immigration offenses as a proxy for terrorism charges. The U.S. would do well to adopt the English detention practice. It allows government to move rapidly, and it is reasonable (reasonableness being the touchstone of our Fourth Amendment).

The U.S. should emphatically not adopt the British practice on wiretapping, even though it, too, also appears to have played an important part in disrupting the airplane plot. Much has been made of the fact that British eavesdropping authority is easier to obtain: The Brits need not go to a court; a high-ranking government official may order monitoring. Nonetheless, England pays a weighty price for this lower hurdle. Wiretap evidence, which is the backbone of most major American criminal cases, is not admissible in any British prosecution, terrorism or otherwise. Essentially, the British have elevated intelligence issues--specifically, the concern that admitting electronic-surveillance evidence in court would compromise intelligence methods--over the need to build strong cases. The result? The Brits may be more apt to stop an attack, but they are far less likely than their American counterparts to convict terrorists and get them sentenced to very long terms that effectively end their terrorist careers. In the long run, moreover, it is dubious that they will actually thwart more attacks: Above all other incentives, it is the threat of prosecution and lengthy incarceration that induces culpable actors to cooperate and provide vital intelligence.

Viable prosecutions will always be a mainstay of any effective terrorism-prevention strategy. Nevertheless, the British success shows that equally essential are commonsense steps that have been relentlessly libeled by privacy-obsessed activists since 9/11. In addition to rapidly establishing telecommunications surveillance (in a way that may not have been possible under the sclerotic standards of the 1978 U.S. Foreign Intelligence Surveillance Act), British investigators monitored money transfers between the plotters--an activity similar to that for which the Bush administration was castigated when the New York Times and other publications exposed its Terrorist Finance Tracking Program. And British investigators reportedly made use of sneak-and-peek search operations--a tactic that came under absurd assault during the Patriot Act debate, but that has long been deemed entirely proper under the Constitution and is critical to government's ability to obtain significant evidence without ending investigations prematurely.

We are reading today about two dozen arrests rather than 2,000 murders because investigative tactics consistent with vibrant civil liberties were adeptly employed. Heading into the fall elections, Republicans would do well to remind Americans that they are the ones who fought to preserve those tools.

Mr. McCarthy is a senior fellow at the Foundation for the Defense of Democracies.