A half-century ago, in the back-to-the-future years after World War II, the cause of preserving the nation's architectural past was high on the priority list of maybe one in a million Americans.

Several of those notable exceptions, however, lived in Washington and were in positions to do something about their opinions. Thus, the National Trust for Historic Preservation was born here in 1949, a small private organization destined to have a major public impact.

The group started with a two-person staff housed in a tiny office up four flights of stairs in the Ford's Theatre building on 10th Street NW. It has grown into today's well-oiled organization of 250,000 members and a large staff comfortably headquartered in a Beaux-Arts building on Massachusetts Avenue NW--once the city's most luxurious apartment house.

One could hardly take a walk downtown this week without bumping into clusters of people with National Trust badges on their lapels, investigating this or that historic building or street. Along with a healthy number of top politicians and high administration officials--first lady Hillary Rodham Clinton heading the list--these individuals were both celebrating the Trust's 50th anniversary and attesting to its enhanced stature.

Since its founding, the Trust has both led and been led by the vast changes that have occurred in the historic preservation movement. Preservation has become more popular, less elitist. Once pariahs, preservationists now are routinely involved in major development decisions.

Success has been great--it is difficult any longer to imagine the American built landscape without the philosophical and practical contributions of preservation. At the same time, it is clear that daunting challenges remain. Preservation has grown in many positive ways, but the movement still defines itself largely in negative terms--by what it is against, rather than what it is for.

Many date the preservation movement's beginning to the 1850s campaign of the Mount Vernon Ladies' Association to save George Washington's home. A century later, the National Trust had similar origins in a successful attempt to save a distinguished private residence.

Founding Chairman David E. Finley, the tactful Andrew Mellon aide who became the first director of the National Gallery of Art, was initially alerted to the need for such an organization while visiting the 18th-century home of a donor to the gallery. Learning that the estate--Hampton, north of Baltimore--was endangered, Finley did what he was uniquely situated to do. He called the Mellons to the rescue. In particular, he persuaded Ailsa Mellon Bruce, Andrew Mellon's daughter, to buy the property and give it to the National Park Service.

"But the process was lengthy and cumbersome and the government regulations frustrating," writes David Doheny in the fall issue of the Trust's journal. Finley thus was attracted to the notion (first advanced by Park Service historian Ronald F. Lee) of establishing an American equivalent to the then-half-century-old British National Trust. Its aim would be to acquire for the public good threatened properties such as Hampton.

With Finley's adept tutelage, the nation's preservation leaders--geographically dispersed, unorganized and few in number--came together at the National Gallery with a few government bureaucrats to recommend such a course. The National Trust was chartered in a law signed by President Truman on Oct. 26, 1949.

The Trust today owns and administers 20 historic properties, but its focus, like that of the movement as a whole, is now much, much broader. In part, this expanding outlook is due to Trust leadership. The Trust lobbied masterfully, for instance, in the writing and adoption of the 1966 National Historic Preservation Act.

This piece of legislation gave preservation a seat at the table. Or, rather, many tables. It established a system of well-defined federal, state and local responsibilities that, faults and all, remains an exemplary demonstration of creative federalism. Trust-supported initiatives that followed, such as various tax packages favoring preservation, increased the movement's clout.

Yet the mettle of the preservation movement as a whole came not from federal laws--as helpful as they turned out to be. Rather, the character of the movement was forged in '60s and '70s battles against massive demolitions caused largely by urban renewal and federal highway construction. The Trust participated in but did not initiate the protests, which were repeated in city after city.

The activist, populist side of the movement had mostly good consequences. It saved buildings and neighborhoods, sharpened preservationist tools (particularly legal ones), increased awareness that "ordinary" buildings could be as important as the estates of the high and mighty, and promoted the idea that entire places were even more important than individual buildings. It educated a new generation of architects to be sensitive to the existing urban context.

But there also were negative effects, and some of these continue to bedevil the movement. Manning the barricades requires a type of certitude that can turn into meddlesome smugness once the fight is over. Many preservationists got so used to glorifying the past and detesting modern architecture that they forgot that change is inevitable and that we live in a modern age.

The adversarial '60s and '70s can perhaps be characterized as preservation's heroic age. The speeches were uplifting, the battles exhilarating. Slow, unsteady progress has been typical of the decades since.

For instance, the Trust's Main Street Program has been a smashing success. Initiated in the early '80s, the bootstrap operation has affected more than 1,500 communities where 61,000 buildings have been renovated and 47,000 new businesses created. The newer Community Partners Program is attempting to produce similar results in distressed urban residential neighborhoods.

Yet so big is the country, and so pervasive are such problems, it is probably safe to say that abandonment and disinvestment continue in America's small towns and city neighborhoods. It has been clear for a long time that preservation, by itself, is only part of the solution. A more comprehensive philosophy and approach are needed.

Happily, the Trust in recent years has been a leading force in the effort to redefine the movement. "We realized we were dealing with the end products of a much larger process," says longtime staff member Dwight Young, "and that process is urban sprawl." Under the leadership of President Richard Moe, a Minnesotan with strong convictions and political skills, unregulated urban growth has become the Trust's Public Enemy No. 1.

To this generation of preservationists, Moe often says, the issue of sprawl is the galvanizing equivalent of yesterday's urban renewal. The problem with "anti" movements, of course, is that the hostility tends to spill over. It is no great distance from anti-sprawl to anti-new, anti-change, anti-almost-everything. But such a step is not automatic, and Moe and others insist it will not be taken.

The preservation movement has long been stumbling to an awareness that saving the past, though essential, is not the overriding issue. Rather, the big issue is building the future. Preservation has much to contribute, but only as part of a larger vision of the good society that embraces change. What can we go back to is not the question. Rather, it is what can we move toward.