Speculation, often grossly uninformed, filled the radio and TV airwaves immediately after the London attacks last Thursday. Pundits, politicians and so-called experts tried to offer instant analysis based on the fragmentary information available.

You'd think after the Madrid bombings and the crisis in credibility that confronted Spanish officials (they initially blamed the wrong terrorist group for the pre-election train station bombings), the talking heads would have learned to be a bit more circumspect. Based on the banalities filling the airwaves and pages of print, though, they haven't.

So let's go beyond the usual bromides and consider a few serious lessons we can draw from the attacks and their implications for U.S. security:

• We must take the threat of transnational terrorism seriously. Less than four years after 9/11, many were claiming that the United States had overreacted, that the notion of a "war on terrorism" was wrongheaded. London reminds us that they're flat wrong. The murderers behind these attacks are deadly serious; they're trying to kill us, and we must stop them. By any sane definition, that's a war.
• The successful attacks on Britain, a country that takes counterterrorism seriously, are sobering. The plain fact is no country can guarantee that its citizens will never fall prey to terrorism. Killing innocent civilians is relatively easy for anyone with the stomach for the job. We have to recognize that no government can guarantee total security. And we can't remove every risk without fundamentally compromising the liberties and destroying the free flow of goods, peoples, services and ideas that sustain free-market economies.
• Terrorists have limited means. They can't be everywhere, all the time. As terrible as the London attacks were, by the evening rush hour the city was on the move again. By the next morning, most of London's transit system was up and running. A few cannot stop the determined many.

Beyond these basic realities there are probably not a lot of immediate lessons for U.S. homeland security. Europe is a different kind of target than the U.S. Transnational terrorist groups have a well-established presence in many European countries, as witnessed not just by the London attacks but also the arrests over the last few years in Britain, Spain, France and Germany.

With that in mind, some things clearly should not be done:

• Don't change the strategy. The U.S. strategy is focused on staying on the offensive, breaking up terrorist networks and pre-empting operations before they threaten us. In addition, recognizing that even aggressive counterterrorism efforts, as the London attacks remind us, won't stop every terrorist, the nation needs layered security as well as a judicious mix of protective and response efforts. This approach has made the U.S. a tougher target for the terrorists.
• Don't throw money at the problem. Already we hear shrill cries for more spending on mass-transit security. Dumping federal dollars on the "danger of the day," however, won't make us much safer. Turning America's subways into a new Maginot Line would cost billions and take forever. And for what? The terrorists would simply attack something else.

Certainly, some prudent measures can be taken to improve mass-transit security, enhance awareness, ensure effective information and intelligence sharing, secure sensitive sites and assets, and boost emergency response capabilities. But most of these efforts are already under way.

There will be many lessons from London. We should benefit from them, but we shouldn't let them shake our resolve or confidence that we're on the right course and will prevail in the war on terror.

James Carafano, co-author of "Winning the Long War: Lessons from the Cold War for Defeating Terrorism and Preserving Freedom," is a senior research fellow at The Heritage Foundation (heritage.org).