In the hours following Tuesday’s attacks in New York and Washington, two emotions dominated: horror at the appalling scale of the carnage, and astonishment at the terrorists’ sophistication.

On reflection, “sophistication” is an inapt term. The hijackers carried no complex electronic gear or advanced explosives. Their only weapons were decidedly lo-tech: plastic knives and box openers of the kind available at any office-supply store.

The hijackers brought to their murderous task a willingness to kill themselves and an impressive level of co-ordination and timing. There is nothing new or startling about those attributes.

What set them apart was a simple idea: the conversion of commercial airliners into weapons, into flying bombs capable of taking out the most prestigious symbols of American military and commercial power.

It’s one of those ideas that seems inconceivable only until the moment someone conceives of it, whereupon it instantly becomes incredibly obvious.

With that simple idea in hand, the worst terrorist act in U.S. history became relatively straightforward: a matter of lining up a few zealots, buying them instruction at commercial flying schools, and equipping them with box cutters, plastic cutler, and airplane tickets.

The scale of the ensuing violence should not blind us to the simplicity of the preparations. We don’t have to, and we should not, conjure up some omnipotent enemy capable of bringing the world’s mightiest power to its knees.

What we’re dealing with here is apparently a small band of murderous fanatics who exploited the world’s failure to anticipate a simple plan of almost unimaginable destructive capacity.

Once anticipated, the idea of converting airliners into weapons becomes a relatively easy problem to counteract. It will not require the technological might of a military-industrial superpower. It will require only thinking, planning and careful attention to detail.

“We do need to improve security, but we should not understand the need for heightened security in a broad, vague way as a cultural imperative,” wrote Phil Agre, professor of information studies at UCLA and publisher of the Internet-based Red Rock Eater News Service.

“We do not need a police state, and we should not militarize our society,” Agre wrote. “Rather, we should view security as a design problem.”

As a design issue, the problem of terrorists turning airliners into weapons seems relatively simple to solve.

As a first step, the door between the cabin and the cockpit of an airliner could be eliminated. Without access to the pilots, hijackers might be able to divert a plane to another destination, but they could not conceivably force it to crash into a designated target.

The operation of a plane’s transponders, which assist ground controllers in tracking an aircraft, could be placed beyond reach of anyone on board. In Tuesday’s attacks, the hijackers were apparently able to disarm them on all four commandeered planes.

The days of entrusting pre-boarding security checks to private firms employing minimum-wage inspectors could end. Those who carry out pre-flight checks could display a level of training and diligence commensurate with the importance of their task.

With careful thought, additional steps can be found to eliminate the danger of terrorists reprising Tuesday’s attacks. The important thing is to concentrate on concrete steps directly focused on the goal of preventing terrorism.

The danger, in light of the understandably bellicose mood sweeping the United States, is that security precautions will go beyond concrete, goal-oriented steps into the promotion of generalized control and authority.

To the extent that the hijackings were an assault on freedom, such a climate would spell victory for the terrorists. You cannot defend freedom by eliminating it.