Since 9/11, the emblem of airport safety has been the security line. Hordes of travelers queue up with their carry-on baggage, and everyone is scanned for bombs and guns and knives and GI Joe toys and large tubes of toothpaste and other tools of terror. Then they can head to their departure gates feeling secure.

Those screeners don't do a good job of catching actual weapons when undercover officers smuggle them through as a security check. (When such tests were conducted last year, the TSA had a failure rate of 95 percent.) But even if the system perfectly protected the planes and gates from attackers, critics have regularly raised another question: Couldn't a terrorist target the line itself? Or, more broadly, the vast area on the pre-entry side of the security perimeter?

That's what happened today in Brussels, where one of this morning's attacks killed at least 14 people at an airport departure hall and injured many more. Someone could certainly do the same thing in the U.S. So what do you do then? Move the queue outside? That merely moves the target again. If an ISIS sympathizer or a neo-Nazi or a neo-Weatherman or just your average alienated asshole of the Adam Lanza variety decides to attack at the curbside instead, there's not much to physically stop him.

It isn't the TSA that has protected the check-in zones of America's airports. It's the fact that there just hasn't been anyone willing to attack them. Terrorists are relatively rare in the United States, and the ones we do have often prefer to strike in other sorts of places. If you could somehow put a security check at every door in America, you might succeed at bankrupting the country and at disrupting the movement that makes society possible, but you wouldn't snuff out the possibility of a terrorist assault. There is no such thing as perfect security, and anyone who tells you otherwise is selling you snake oil.