Invasion is undoubtedly as old as humanity. Hunting groups must occasionally have encountered each other, if only by accident. Each side must have felt the other was invading. The obviously weaker side would have had to decamp. If the matter were not obvious, there might have been threats or even a brief struggle to settle the matter.

Once agriculture became a way of life and farmers were pinned in place by their farms and food stores, these same food stores became an overwhelming temptation to surrounding nomads; invasions were more terrible because farmers could not flee but had to stand and fight.

We begin to have records of early civilizations suddenly inundated and taken over by raiders. The Sumerians were taken over by Gutian invaders as early as 2200 B.C. The Egyptians fell under the grip of the Hyksos invaders soon after 1700 B.C. We can go through an endless list of such things.

Considering that those people who were invaded (until quite recent times) had little knowledge of the world outside the boundaries of their own cultures, the invasions must usually have come as unbelievable shocks, as a sudden influx of the unknown from the unknown. This would be especially so when the invaders spoke strange languages, wore strange clothes, had strange ways, and even, perhaps, have looked odd.

As the most recent example of our cultural ancestors being subjected to the horror of an unexpected invasion, we need only go back to 1240, when the Mongols (short, squat, slant-eyed) swept into Europe on their hardy desert mounts. Europe knew nothing about them, had no way of knowing they were on the way (they had been ravaging Asian kingdoms for twenty years). All they knew were that these terrible horsemen, moving with incredible speed and organization, winning every battle, smashed Russia, Poland, Hungary, and were penetrating Germany and reaching for Italy, all in a matter of a single year. And then they left and raced eastward again, smashing Bulgaria en route. (They left because their khan had died back in Mongolia and the army had to be there for the election of a successor. Nothing the Europeans could have done would have stopped them.)

But the Mongols were "the last of the barbarians." Partly because of the Mongolian empire that was set up, communications between China and Europe became smoother. Such things as printing, the magnetic compass, and (most of all) gunpowder, leaked westward from China, and these things-for some reason not exploited by the technologically more advanced Chinese-were put to amazing use by Europeans.

And beginning about 1420, the tide of invasion was reversed. The "civilized" Europeans, with their ships and their guns, fell upon the coastlines of all the continents and, eventually, penetrated the interiors until Europe dominated the world politically and militarily (and as it still does, even today, culturally.)

But how did the non-Europeans feel about it? How about the Africans who watched the Portuguese ships come from nowhere and carry them off as slaves; the Asians who watched Portuguese, Dutch, and English ships come in set up trading posts, skim off profits and treat them as inferiors; the Native Americans who watched the Spanish ships come in and take over and destroy their civilizations? There must have been the feeling of monsters arriving from some other world.

All invasions, however, at least of the kind I'm discussing, were by human beings. However strange they might have seemed-Mongols to Europeans, or Spaniards to Incas-they were clearly human beings. (There were also invasions of infestations of non-human types-rats, locusts, the plague bacterium of the Black Death, the AIDS virus-but these fall outside the subject matter of this introduction, and even they were forms of terrestrial life.)

What if, however, the invaders were intelligent beings who were not human and, in fact, not Earthly. The possibility did not seriously arise until the time when it was thoroughly recognized that the planets were other worlds and that the universe might be full of still other planets outside the domain of our own sun.

At first, other worlds were the subject of "travel tales." Human beings went to the moon (as early as the second century A.D. in fiction and more frequently as time went on), but there are no tales I can think of in which the inhabitants of the moon came to Earth.

In 1752, the French satirist Voltaire wrote Micromegas, in which visitors from Saturn and Sirius observe the Earth, but this cannot be taken literally. The visitors are merely Voltaire's device for having Earth viewed with apparent objectivity from without in order to have its follies and contradictions made plain.

But then in 1877, there was the discovery of thin, dark markings on Mars. This was interpreted by some as "canals" and the American astronomer Percival Lowell was convinced that they were artificial waterways built by intelligent beings trying to use the ice of the polar caps to maintain agriculture on their increasingly desiccated planet. He wrote books on the subject in the 1890s that created quite a stir.

The British science fiction writer Herbert George Wells proceeded to make use of the notion and, in 1898, published The War of the Worlds, the first significant tale of the invasion and attempted conquest of Earth by more advanced intelligences from another world (in this case, Mars). I have always thought that Wells, in addition to wanting to write an exciting story with an unprecedented plot, was also bitterly satirizing Europe. At the time he wrote, Europeans (the British, particularly) had just completed dividing up Africa without any regard for the people living there. Why not show the British how it would feel to have advanced intelligences treat them as callously as they were treating the Africans?

Wells's novel created a new subgenre-tales of alien invasion. The manner in which Wells made the Martians unpitying exploiters of humanity (for the sake of excitement and, I believe, satire); the memory, perhaps, of the Mongol invasion; the feeling of guilt over the European despoliation of all the other continents; combined to make it conventional to have the alien invaders unfeeling conquerors, for the most part.

Actually, we have no reason to think this would be so. As far as we know, no invaders from without have ever reached Earth and, for a variety of reasons, it might be argued that none ever will.

However, if they do come, there is no a priori reason to suspect they won't come in friendship and curiosity, to teach and to learn.

Yet such is the power of humanity's own shameful history and the conventions of fiction that very few people would be willing to consider alien invaders coming in peace as a real possibility. In fact, when plaques and recordings were placed on rocket probes designed to leave the solar system and go wandering off into interstellar space, in order that alien intelligences (if any) might find them someday, millions of years in the future, and that they might thus learn that Earthmen had once existed-there were those who thought it a dangerous process. Why advertise our existence? Why encourage ferocious aliens to come here in order to ravage and destroy?

Here, then, in this collection, are stories of alien invasion. We have selected a variety of contemporary treatments of the problem, some a matter of excitement, some thoughtfully philosophic, some even funny. They view the possibility from all angles and stretch our minds on the matter, as good science fiction should.