The most sought-after tour guides in Washington, D.C., are White House staffers. Beginning at 7:30 P.M. on weeknights—unless the President is still at work in the Oval Office—and on weekend afternoons and evenings, press aides and policy advisors can be found leading family members and friends, along with well-connected tourists, through the West Wing. Few staffers will admit it, but they cram for these tours as they would for a college history exam, and come prepared to tell you why the Roosevelt Room used to be called the Fish Room (or, by F.D.R., “the morgue”), and where Nixon’s taping system was installed, and why the eagle on the front panel of the President’s desk and the eagle on the ceiling of the Oval Office face in opposite directions.

During the Clinton Administration, when I gave many of these tours, I would end by escorting my guests out of the West Wing, past the Press Briefing Room, and through an entryway under the North Portico. There, just beneath the front door, is an archway that still bears scorch marks from the British burning of the White House in 1814—singed stone that offers blunt witness to the vulnerability of the building and its tenants. Visitors could see plenty more evidence of that: iron gates, concrete barriers, guard dogs, bomb-sniffing dogs, rooftop sentries, and, on the north façade, near a second-story window, pockmarks left by a gunman who, in October, 1994, pulled a semiautomatic rifle from his trench coat and fired multiple rounds through the White House fence, just yards from where Omar Gonzalez made his leap, last month, into public consciousness.

Tuesday’s hearing before a House oversight committee—during which Julia Pierson, the director of the Secret Service, failed to pacify members with her assurance that Gonzalez’s self-guided tour of the state rooms with a knife on him was the sort of breach that “will never happen again”—revived some very old questions about securing the People’s House, which also happens to be the President’s home. Despite the shocking laxity of the Secret Service, which, it was revealed yesterday, had no idea that a contract security guard who rode an elevator with President Obama in Atlanta last month was carrying a gun, no one is under any illusion about the manifold threats the White House faces: from Pennsylvania Avenue, which runs a hundred and eighty feet from the North Portico; from Constitution Avenue, farther but close enough to hit with bullets, as an assailant did in 2011; or, indeed, from above.

The White House has fortified itself by degrees. L’Enfant’s original plan for the site, in the eighteenth century, followed the European model: a pallace at the center of expansive, and therefore protective, grounds. The house we got instead—scarcely a quarter of the size of L’Enfant’s design—was a better fit for a young republic, but was dangerously exposed from the start. Monroe, afraid of assassins, put an iron fence around the perimeter and sharpshooters in the trees. Lincoln, during the Civil War, refused to make the mansion a garrison, but made quiet concessions to reality: plain-clothed sentinels patrolled the property. During the Second World War, F.D.R. closed West and East Executive Avenues, which bracket the building, though he rejected a proposal by a security panel that he foil nighttime air attacks by painting the White House black.

And so on and so forth: Fences grew higher and more imposing (if not imposing enough), gates were closed, windows were filled with bulletproof glass, walls were reinforced, security checkpoints were installed. In 1983, concrete barricades went up. In 1995, a month after the truck bombing of a federal building in Oklahoma City, President Clinton yielded to the Secret Service and to his Treasury Secretary, Robert Rubin, and closed two blocks of Pennsylvania Avenue to vehicle traffic. And today, in what can only be seen as a symbolic response to the Gonzalez incident, the ornamental fence has now been seconded by a hastily built, sectional metal fence, draped with yellow caution bunting, giving the White House an additional five feet of breathing room...