It turns out the Secret Service isn't too good at protecting the White House, and maybe one reason is that we don't want it to be.

Secret Service agents are famously willing to sacrifice their own lives to protect the president and his family. They are also trained to take the lives of others in defense of their protectees.

But are they equally prepared to do either of those things for the White House itself? Should it be policy for the armed agents around 1600 Pennsylvania Avenue to use deadly force whether the president or his family is present or not?

Most Americans see the White House as a symbol of the nation, like the Capitol or the flag. Most do not realize how exposed the physical reality of that symbol is, situated in the center of a major urban metropolis with an antiquated security fence just yards from the front door.

It is surely possible for the Secret Service to shoot anyone who jumps or squeezes through that fence, but in recent months that would have included at least one errant toddler — whose story was told in the media as a cute back-page "bright."

It is also surely possible to electrify the fence or its immediate vicinity, but that would very likely lead to incidents of an unpleasant nature — and all the predictable reaction in the media and beyond.

In either event, the Secret Service would be pilloried as either inept or trigger-happy. The president would be portrayed as besieged, unfeeling, remote. Even the signs on the fence warning of lethal consequences would be a ghastly image.

In 1995, a truck bomber in Oklahoma City killed 168 people and leveled a major federal building. In response, the Secret Service succeeded in closing Pennsylvania Avenue to vehicles, lest a copycat park a truck within yards of the North Portico of the White House.

But even now, it is possible for pedestrians to get close enough that a sprinter can cross the grass and enter the building. That is what 42-year-old Omar Gonzalez was able to do on a Friday night. The Washington Post revealed this week that Gonzalez got to the Green Room on the ground floor before being subdued. That contradicted earlier reports of his being stopped in the entryway.

All this led to the resignation of Secret Service Director Julia Pierson, following an animated House hearing Tuesday on the incident. Member after member denounced the officials of the agency, proclaiming their shock and dismay. Surely they spoke for millions of their constituents, who usually rank the Secret Service among the federal agencies they are most inclined to trust.

Yes, the agents could have shot him. They also could have released trained dogs that might have taken him down. But that would have meant an ugly story about the treatment of a man carrying nothing more threatening than a knife, as was noted at the hearing by former Secret Service Director W. Ralph Basham.

"We could be here having a very different conversation," said Basham, making the point that decisions about security at the White House were not and had never been the province of his agency alone.

The Secret Service has never been able to guarantee the safety of the president. That point has been stated and restated countless times. Anyone willing to exchange his own life for that of the president has always had, and still has, at least a theoretical chance of succeeding.

The recent revelations about the jumper and of an unknown gunman hitting the White House in November 2011 are disturbing and should prompt a review of agency policy and practice — and some soul-searching on the part of all concerned.

But there are also sound reasons behind the reluctance to build an impregnable White House. Draconian measures such as closing Pennsylvania Avenue to pedestrians or barricading Lafayette Square park across the street would be undesirable and unattractive — offensive to the national attitude toward the White House as "the people's house."

The president obviously must be protected. But what politician wants to be seen as living within a fortress in a state of siege?

In ancient Roman times, top generals hand-picked a few legionnaires to guard their own personal headquarters. The HQ was called a praetor, and the protectors became known as praetorian guards. These elite units grew in size and importance until Caesar Augustus made them his official protectors.

Over the next three centuries the praetorians became a key element in Rome's recurring power struggles, sometimes protecting emperors and sometimes assassinating them. Since then the term praetorian has been used to connote a protective inner circle that either grows too powerful or otherwise becomes a problem.

The Secret Service is surely a far cry from such a force. But allowing armed guards all the leeway they might require to do their job perfectly can have unintended consequences as well.

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