One hundred and fifty-six nations will unveil their pavilions next Thursday at the beginning of Expo 2000 in Hannover, Germany--the first full-scale world's fair in eight years.

But the United States will not be there.

The absence is glaring, the comparisons uncomfortable. Japan asked one of its leading architects to design a multistory building out of recycled paper. Gardeners from the Netherlands planted a "floating landscape"--full-size trees included--on the open-sided fourth floor of a six-story pavilion. Venezuela's big glass box will feature more than 10,000 tropical plants. Ethiopia's pavilion is modeled after a mountain village. That of Bhutan resembles an elaborate Buddhist temple.

Expo 2000, in other words, is likely at the very least to meet world's fair norms for feats of design and showmanship. It is a great international get-together, rallied around the theme of global ecology.

Inside the pavilions, large and small, nations or international organizations will exhibit their vitality, inventiveness, creativity. There will be a lot of face-to-face contact--40 million visitors are expected at Expo 2000--and a lot of information will get exchanged.

But from the United States in Hannover there will be nada, zip, zero, zilch.

This is all the more regrettable because of the might-have-beens. When German authorities were laying out the vast fairgrounds a few years back, they offered the United States one of the largest and most prominent sites. The invitation was happily accepted.

When potential European visitors to the fair were asked in a marketing survey to name the pavilion they would most like to visit, the United States came in first. At the time, promoters of the U.S. pavilion touted these gratifying results.

Yet the pavilion was not to be.

It is hard to say exactly why. The short answer is, no money. For the richest country in the world, that would seem to be a joke but, in a narrow sense, it happens to be true.

William D. Rollnick, commissioner general of the U.S. exhibition at the fair, was faced with the task of raising $45 million in the private sector to pay for the pavilion. He could not do it, and that was that. The U.S. officially folded its world's fair tent last month.

There were serious signs of trouble all along. Rollnick--a Mattel director and big-time Democratic campaign contributor--volunteered for the unpaid post at the request of his friend, President Clinton. He got off to an ambitious, if late, start.

In the spring of 1998, Rollnick and his wife, photographer Nancy Ellison Rollnick, who served as his deputy commissioner, asked a New York architecture firm to come up with an idea for the pavilion. It was a great choice and a promising idea: James Wines of SITE (an acronym for Sculpture in the Environment) proposed a "tapestry of America" in a couple of wavy-edged buildings separated by a winding path called "Route 66."

Under the twin pressures of time and money, however, Rollnick changed architects and announced a simpler design in the fall of 1998. But even the stripped-down design proved to be too costly. American corporations were not responding to the appeal, and Rollnick, U.S. Ambassador to Germany John C. Kornblum and fair organizers were left scrambling for alternatives.

In the end, nothing worked. More than one observer sarcastically suggested that the United States apply for one of the grants Germany was giving to poor countries to help with their exhibitions at the fair. (For the record, this "development fund" totaled about $50 million.)

That the private sector did not come forth with the dough is not so surprising. Many corporations these days are so global they don't want to be identified with a single country. Many are so wealthy they can, if they want, saturate a fair with advertising. Why play second fiddle?

What is surprising, and disconcerting, is that it has come to this. World's fairs are part of history. Often they are remembered for an outstanding building or symbol--the Eiffel Tower in 1889, the Trylon and Perisphere in New York in 1939. The giant fairs tend to lift the spirits and, not incidentally, improve the cities that host them--ancient Seville, Spain, made itself ready for the 21st century in preparing for the fair in 1992.

The United States has been present at every major fair since the first one, held in London 149 years ago. In the post-World War II era--at Brussels in 1958, Montreal in 1967 and Osaka in 1970--our pavilions have been enormously popular and aesthetic hits. We asked some of our best architects and designers to do the job, and they did.

It was at the Seville fair that we began seriously to falter. Our pavilion there was a bad joke--a couple of well-traveled, medium-size Buckminster Fuller domes (tepid reminders of his thrilling 1967 Montreal design) and super-scale Peter Max graphics.

The process was terribly flawed, as well. Again, there was an ambitious beginning, with an architectural competition among several of the best talents in the land. But a fine design from that competition got so whittled down by cost cuts and other changes that the architect rightly refused to let his name be used in connection with the final product.

The Seville pavilion was billed as a public-private partnership, but it ended simply as a public fiasco. After Seville, money got even tighter: In an obscure paragraph deep within a foreign relations appropriations bill, Congress in 1994 made it almost impossible to get any government funding for such international exhibitions.

If the language seems innocuous--the law simply says such expenditures have to be specifically asked for, authorized and appropriated--the effect has been dead certain. No money has been requested, and none given. What had been mainly a public responsibility for more than a century, and then had been changed briefly to a joint public-private enterprise, was transformed almost entirely into a private obligation.

As our Hannover failure makes painfully clear, this system doesn't work.

And why should it? Representing the United States abroad--the whole United States in all of its glorious diversity--is not what private companies were put on this Earth do to.

There are exceptions, I suppose. The Amway Corp. fully funded a U.S. presentation at a smaller fair in Taejon, Korea, in 1993.

But even that generous, public-spirited gesture leaves hanging a basic question: How many of us can be comfortable when Amway--or any other corporation--undertakes in the name of us all to represent American culture in faraway lands?

It seems clear, on the other hand, that adequately--or even superbly--representing the United States at those fascinating occasional events called world's fairs is not on anybody's agenda in Washington. Some on Capitol Hill obviously think such events are a waste of tax dollars, although this definitely is a low-key issue, way down at the bottom of the Washington barrel.

Those who might be expected to see things differently, to take a more cosmopolitan, more expansive view, somehow don't do so. The last two presidents have simply gone with the flow--the cause perhaps seems too small to fight for.

The bureaucracy doesn't care. The question seems to bring trouble, and who wants trouble?

Many in Congress might be embarrassed by our no-show in Hannover, but it isn't exactly an issue constituents will flood the phones for.

Yet leadership is sorely in demand. The world, in a sense, is going to Expo 2000, and the United States is staying home.

Is that the message we ought to be sending?