But that openness is under siege. The proposed Vietnam memorial visitors' center would need to accommodate more than 4 million people who come to the site every year. Like the African American history museum, its size would be daunting. Think historical displays, educational exhibits, memorabilia collections, toilet facilities and more. Even a building constructed underground would require entries, exits, ventilation systems, skylights -- intrusions above ground that chip away at the Mall.


Approved or proposed Mall additions now include memorials to President Reagan, Martin Luther King Jr., terrorist victims, Native Americans and soldiers lost in peacekeeping missions. Then there's the network of tunnels, underground security checkpoints and surveillance cameras newly suggested for the Washington Monument. Like other burgeoning examples of a post-Sept. 11 "architecture of fear," these schemes would destroy the monument in order to save it.

Many memorial planners were doubtless emboldened in 2001 by the success of backroom maneuvering in getting the huge, 17-acre World War II Memorial a prime location on the Mall, right between those to Washington and Lincoln, where construction is now underway. That disastrous design, based on foreign U.S. military cemeteries, jams a funereal stake into the splendid Enlightenment heart of the L'Enfant-McMillan scheme. The World War II Memorial is a monumental act of hubris -- not a proud statement of founding principles like the other five structures, but a medal of honor pinned with egoistic pride on the national chest.

It also shredded the Commemorative Works Act of 1986, which Reagan signed into law in response to just such threats to the Mall's magnificent open space. The law was a safeguard meant to counter the emotionalism of any memorial process. But in the aftermath of the Cold War, sentimental appeals to the passing of the so-called "greatest generation" proved too strong. An exception was made.

Supporters of the Vietnam memorial addition have apparently learned the lesson. With American troops now in harm's way in Iraq, they've made their move. This is the third time legislation has been introduced for the ill-advised project, and it may well be the charm. The first two didn't have a tide of patriotic enthusiasm on which to ride.

OFF-SITE IDEA

Some congressmen do want to strengthen the Commemorative Works Act -- an essential but arduous step. An amendment to last year's failed legislation authorizing a Vietnam memorial education center had the right idea. It correctly described the Mall as "a substantially completed work of civic art" and prohibited new projects in the park's great cross-axis. Supporters of a Reagan memorial -- likely to exploit the high emotion sure to follow the death of the conservative idol, now 92 -- killed the bill because of it.

A useful idea has been offered by the National Coalition to Save Our Mall, a scrappy citizens' group that rose up to fight the World War II Memorial fiasco. For if an education center is warranted at the Vietnam memorial, what's to stop others from being built there and at the Korean War Memorial?

The coalition suggests establishing one off-site visitors' center for the entire Mall. It could encompass a national military museum, not unlike London's Imperial War Museum, where all conflicts in defense of the nation could be explained. Historic military installations line both sides of the Potomac and Anacostia rivers, and several would make an ideal museum site.

It might also explain the powerful historic meaning of the Mall's open space, not least to Congress. But if politicians won't protect the Mall, the people must. It's time to sound the loudest general alarm.

The World Monuments Fund -- the foremost private nonprofit organization dedicated to the preservation of historic art and architecture worldwide -- should be petitioned. The fund's current list of the world's 100 Most Endangered Sites ranges from Egypt's ancient Valley of the Kings to the pivotal 1921 Schindler House on Kings Road in West Hollywood. Two imperiled cultural heritage sites in Iraq are listed, while the historic neighborhood of Lower Manhattan joined in the wake of Sept. 11.

The fund can't stop the destruction, but it can let the world know what's at stake. The richly empty 2-mile stretch of the National Mall is the most important cultural icon in all of America's history. But slowly, they're paving it over.

Christopher Knight is The Times' art critic.