It happened here, too. The British burned our national library in 1814. It wasn't much of a library, to be sure -- just a collection of about 3,000 volumes assembled for the use of senators and representatives in the new capitol being built in the wilderness of Washington, D.C. But in destroying it, the British invaders struck at the heart of what would develop into a national identity.

Do libraries really matter for a nation's sense of its self? Evidently Iraqis felt the destruction of their national library, archives and museum in the past week as a loss of their connection to a collective past, something like a national memory. When asked to explain what the National Museum of Iraq had meant to him, a security guard answered, in tears, "It was beautiful. The museum is civilization." Even some of the looters are reportedly beginning to return what they had carried off, as if in response to a need to heal a self-inflicted wound.

The great collections in Baghdad bore testimony to the beginnings of what much of the world views as civilization. Some of its treasures were 7,000 years old, and they provided evidence about the earliest and perhaps the greatest achievement in human history, the invention of writing, somewhere between the Tigris and Euphrates 5,000 years ago. True, the damage may have been less than was feared at first, and archaeologists can study other clay tablets dug up from the ruins of the world's first libraries, the Sumerian temples of ancient Mesopotamia. But nothing remains of Iraq's National Library, which was burned to the ground along with the Ministry for Religious Affairs and its priceless collection of Korans, some of them more than a thousand years old.

The library burned by the British in the War of 1812 was four years old. Yet its loss was a national trauma, or at least so it seemed to Thomas Jefferson, who had a powerful sense of what libraries could contribute to the civic spirit of the nation. Already, in 1791, he had deplored the damage inflicted by the Revolutionary War on the historical record of America. In a letter to Ebenezer Hazard, who was about to publish two volumes of state papers from the colonial archives, he wrote:

"Time and accident are committing daily havoc on the originals deposited in our public offices. The late war has done the work of centuries in this business. The lost cannot be recovered, but let us save what remains: not by vaults and locks which fence them from the public eye and use in consigning them to the waste of time, but by such a multiplication of copies, as shall place them beyond the reach of accident."

As soon as he learned of the loss of Congress's first library, Jefferson offered to sell it his own, which was twice as big, a magnificent collection of 6,487 volumes that would be valued conservatively at $23,950. The proposal provoked some partisan oratory about the "finery and philosophical nonsense" -- much of it French -- that Jefferson had collected, and it passed Congress by a margin of only four votes. But the Library of Congress stands today as the embodiment of our national memory. Imagine a horde of vandals burning it and the National Archives while an alien army guarded the FBI headquarters and the Treasury Department, and you may have some notion of how Iraqis felt when American troops erected a protective cordon around the ministries of oil and of the interior while permitting looters to demolish the National Library and ransack the National Museum. As many have remarked, the Mongol invasion of 1258 resulted in less damage to Iraqi civilization than the American invasion of 2003.

Jefferson was right. National libraries and museums provide the material from which national identities are built. There are other sources, too -- myths, ceremonies and the other forms of culture studied by anthropologists. But complex societies have been through so much that their history requires constant reassessment. Destroy the documents, and you will damage the collective memory, the sense of self that derives from the ties that bind a people to their ancestors. Libraries and museums are not temples for ancestor worship, but they are crucial for the task of knowing who you are by knowing who you were. That kind of knowledge must be continuously reworked. Destroy the possibility of replenishing it, and you can strangle a civilization.

The most famous case is the ancient library of Alexandria, one that supposedly aspired to include every book in the world -- that is, the Hellenistic world from the third century B.C. -- and whose destruction signaled the end of the world of antiquity. Difficult as it is to disentangle the facts from the myths surrounding the library's history, a few points seem clear: No, Mark Antony did not woo Cleopatra by giving her the rival library of Pergamum, nor did the collection in Alexandria at its zenith reach 900,000 papyrus rolls, although it represented the greatest stock of learning available anywhere in the Roman Empire. Julius Caesar did not burn it to the ground in 47 B.C., and the Muslims did not finish it off in a fit of fanaticism after conquering Alexandria in 642. It probably had disintegrated long before that, not from violence but from the rotting of the papyrus. In short, the library of Alexandria did not come to a dramatic end in a way comparable to the National Library in Baghdad.

But burning and looting has marked the history of libraries at crucial turning points, beginning with the sack of Athens in 86 B.C., when the Romans carried off the remains of Aristotle's library, the greatest in Greece and the model for the library of Alexandria. In the latest study of the Alexandrian library, Luciano Canfora invokes a series of catastrophes -- Athens, Rome, Pergamum, Antioch, Constantinople -- and concludes sadly: "By the middle of the fourth century, even Rome was virtually devoid of books. . . . Surveying this series of foundations, refoundations and disasters, we follow a thread that links together the various, and mostly vain, efforts of the Hellenistic-Roman world to preserve its books." The loss of the books meant the loss of a civilization. Classicists have been able to piece together pictures of antiquity by picking through the remains, but we probably know only a small fraction of what we might have known, had the libraries survived.

The obliteration of civilizations cannot be confined to the remote past, where we can deplore it at a safe distance and in an elegiac mode:

To the glory that was Greece

And the grandeur that was Rome.

Vandals hack away at cultures all the time. They are doing so today in the jungles of Central America and Southeast Asia. Vast stretches of civilization disappeared irrevocably a few years ago when the libraries of Sarajevo and Bucharest went up in smoke. And the Khmer Rouge may have wiped out much of what can be known about Cambodia's civilization when they destroyed most of the contents of the National Library in Phnom Penh.

That in fact was the goal of Pol Pot's army, to obliterate the past and start anew at what they called "Year Zero." Not content with burning the books (at least 80 percent perished), they also killed the librarians (only three of 60 survived). The most valuable books were inscribed on palm leaves. Since the leaves decay in tropical humidity, they had to be recopied every few years by Buddhist monks. But the Khmer Rouge also destroyed the monks, so there was no one left to save what remained of the library.

Perhaps the Cambodians can overcome the trauma by turning it to their advantage, as if to say, "Very well, we shall begin again at ground zero, and now we will build something new." Fresh energy of that kind was generated by some of the destruction of the French Revolution. The Bastille was not merely stormed but dismantled, and its stones were sold off as relics of despotism, remnants of a culture to be replaced by a new political order. Something of the sort could happen in Iraq -- but how? How will the Iraqis fuse a national identity out of the diverse cultures that have come apart with the destruction that has robbed them of their common past?

Few people appreciate the fragility of civilizations and the fragmentary character of our knowledge about them. Most students believe that what they read in history books corresponds to what humanity lived through in the past, as if we have recovered all the facts and assembled them in the correct order, as if we have it under control, got it down in black on white, and packaged it securely between a textbook's covers. That illusion quickly dissipates for anyone who has worked in libraries and archives. You pick up a scent in a published source, find a reference in a catalogue, follow a paper trail through boxes of manuscripts -- but what do you discover in the end? Only a few fragments that somehow survived as evidence of what other human beings experienced in other times and places. How much has disappeared under char and rubble? We do not even know the extent of our ignorance.

Imperfect as they are, therefore, libraries and archives, museums and excavations, scraps of paper and shards of pottery provide all we can consult in order to reconstruct the worlds we have lost. The loss of a library or a museum can mean the loss of contact with a vital strain of humanity. That is what has happened in Baghdad. But when confronted with the loss, Defense Secretary Donald H. Rumsfeld appeared to be unperturbed: "We've seen looting in this country," he explained at a Pentagon briefing. "We've seen riots at soccer games in various countries around the world."

Next question. Robert Darnton is the Shelby Cullom Davis Professor of European History at Princeton University.

An Iraqi collects materials from the destroyed National Library in Baghdad on Thursday.