The Jordan books are indicative of how the Tolkien-inspired universe has changed since the 1940's, and of what the essence of its current appeal may be. After all, why should fantasy novels take place in societies that seem medieval? Why the thatched huts and stone castles, the hand-to-hand combat? Why the wizards and esoteric masters of magical power? And why has this become so distinctive a genre for the late 20th century?

Tolkien was a medieval scholar and philologist who had mastered Old English, Old Norse and Celtic languages, and he set himself the task of inventing not a literary genre but a lost language; that language would then provide the essence of a world, giving it its flavor, its myths, its conflicts. He wanted ''The Lord of the Rings'' to sound like a translation of a medieval epic romance originally written in a foreign tongue. Moreover, the spirit of the medieval romance, in Tolkien's case, also had a national significance, as the scholar Norman F. Cantor has shown. Tolkien fought in World War I and began writing his fantasies on the eve of World War II. Their world bears the marks of that experience and resonates with threats to England's heritage and the prospects of its decline.

Indeed, that world has already fallen before the books begin; it has already lost an ancient wisdom. It is full of ruins, allusions to lost powers, reminiscences of glorious kingdoms. But it is also on the brink of still greater disaster, in which Darkness threatens a final battle. A victory will not bring a restoration but will usher in a different age -- a postwar universe -- with new laws and pains.

This is almost exactly the situation in Robert Jordan's series. And Mr. Jordan's personal history in some ways seems an American echo of Tolkien's. Mr. Jordan, who lives in South Carolina, was educated at the Citadel and fought in Vietnam, earning the Distinguished Flying Cross and other honors. The books' battle scenes have the breathless urgency of firsthand experience, and the ambiguities in these novels -- the evil laced into the forces of good, the dangers latent in any promised salvation, the sense of the unavoidable onslaught of unpredictable events -- bear the marks of American national experience during the last three decades, just as the experience of the First World War and its aftermath gave its imprint to Tolkien's work. And Mr. Jordan also creates a world where a great deal of lore and knowledge is already forgotten and much that exists is badly scarred.

Tolkien and Mr. Jordan are not alone in their visions of postwar societies. The entire fantasy genre is preoccupied with the nature of nation-building and restoration, a subject that can seem far more central than the omnipresent magic and wizardry. In an often fascinating novel, ''Tigana,'' by Guy Gavriel Kay, for example, the focus is on the plots and counterplots of political revolution and court confrontations. In a series of books about a Mormonesque prophet named Alvin Maker, Orson Scott Card attempts to provide an alternative history for the United States: folk magic is as real as the demonic forces hiding out in church, state and family. In her best-selling novel ''The Mists of Avalon,'' Marion Zimmer Bradley draws on one of the touchstones of the fantasy genre -- the Arthur legend -- telling it from the point of view of its women; at stake is the destiny of Britain.

There is also some resemblance to the popular 19th-century novel, which was often concerned with the origins of nations: Sir Walter Scott and Alexandre Dumas turned to Scottish legends and the French court for their wide-ranging tales. And, as in 19th-century novels, fantasy fiction attempts to show destiny unfolding on a large scale, describing societies from high to low, from king to peasant, from clan leader to serf.

It is odd that such attempts at realism should be combined with so thorough a determination to avoid earthly history. But that is part of the genre's point and goes along with its fascination with magic. Arthur C. Clarke once said that any sufficiently advanced technology is indistinguishable from magic, but although technology in fantasy might seem to be an invasion of alien forces, even Tolkien has one of his hobbits see a vision of what would become of his pastoral Shire should the battles against Evil be lost: the charming old mill is replaced by red-brick buildings belching black smoke.