DEATH OF THE FIFTH SUN By Robert Somerlott Viking. 504 pp. $19.95

George Santayana's warning that those who forget history are doomed to repeat it is true as far as it goes, but it doesn't go far enough. After all, those who remember history often remember it selectively, to justify a course of action to which they are already committed.

In "Death of the Fifth Sun," not only has Robert Somerlott written a brilliantly imagined novel, he also provides us with a timely disquisition on the remembering of history. The story of Herna'n Corte's' conquest of Mexico and triumph over the Aztec empire of Moctezuma resonates with the echoes of our own struggles over our proper role in that part of the world.

Lesson: The Aztecs were barbarians who practiced a cannibalistic religion, and their destruction by the Spanish, who brought Catholicism to the New World, was both inevitable and correct. Lesson: The Spanish, who brought smallpox to the New World, were an imperialistic gang who, in their lust for gold and territory, destroyed a noble civilization. It is only part of Somerlott's accomplishment that he rejects both lessons and produces a believable, humane vision of the violent coming of western religion to 16th-century Mexico.

He achieves this through the creation of a narrator who is outside both the Aztec and Spanish cultures. She is Ce Malinalli, daughter of a minor prince who, although commander of a series of small Aztec outposts, is not himself an Aztec. He is from Culhuacan, one of many cities and tribes subdued by the Aztecs in the course of their own bloody empire-building. Malinalli's name, ordained by the position of the stars, is "so unlucky that it can be called truly disastrous" and she is shunned even by her own people. Her father's fate, ordained by Moctezuma, does nothing to further endear the Aztecs to her.

And so, when fate thrusts her into the path of Corte's' army and into the arms of Corte's himself, she is sanguine and becomes his lover and interpreter.

Malina, as she comes to be known, sees in the conquistador the prophesied return of the god called the Plumed Serpent, and swears to serve him. The god has returned many times before over the centuries in many guises: Corte's is just one more. And Somerlott's Corte's is sufficiently complex -- that is, human -- to support Malina's double vision of him as fallible man and immortal god. There are several similarities between the Plumed Serpent and Corte's' God -- both were born of virgins and despised human sacrifice, for example -- and so when Malina translates Corte's' Christian sermons for the natives, she does so in terms understandable and acceptable to them. A skilled diplomat, Malina does not inform the straightforward Corte's of this very useful tactic.

Scorned by the Aztecs as a traitor and distrusted by most of the Spanish, Malina sees through the hubris of both civilizations. With an outsider's clear vision, she learns the only important difference between the Spanish and the Aztecs is that one will win and the other lose. Old virtues will give way to new virtues; modern evils will replace ancient ones, as they always have. Her position between societies protects her from the effects of selective memory: "{The Spanish} had invaded just as {the Aztecs} had once done and Plumed Serpent's {followers} before them. How many yet to come? ... No one owns the earth for long."

She is an excellent, red-blooded creation. One can walk around her and see certainty and confusion, pride and pragmatism. Her graceful, supple voice and tough, ironic sensibility are matches for Gore Vidal's outsiders, while Somerlott's imagination is as glittering and convincing, at home in both the intrigues of Moctezuma's court and the squabblings of Corte's' army.

The reviewer is a writer in New York