It's one of the most well-worn of tropes: aliens invade. They come to Earth. Motherships hover over our cities, fighter-craft with bizarre aerodynamics swarm in our atmosphere, horrific walker-machines pace the ground, or individual aliens traipse the land on foot. They attack with lasers or blasters or even hand-to-hand combat weapons. Or they engineer disease, or send robots to scour the planet. They seek to make slaves or food or raw materials of the human race, or to steal our planetary wealth. Most often, they are defeated through some clever turning of their own technology or biology against them.

Though science fiction is premised more on the scientifically possible than on pure fantasy, alien invasion stories are, as a practical matter, as fictional as any army of orcs summoned by a magic spell, and I will outline two rules which make it so:

Rule Number One: Any alien species technologically capable of travelling from one star system to another would be able to utterly conquer Earth, or effectively wipe it clean of all life, without resorting to trickery, or even coming within range of any Earth defenses.

The energies required to travel from one star system to another in any reasonable length of time can only be harnessed by technologies of the level which would enable its possessor to destroy Earth from afar. It is inconceivable for example that an interstellar spacefaring species would not have long ago discovered the principles of the atomic bomb, and be able to blanket the Earth with them much as nations once feared we would do to ourselves; but with even less fanfare, approaching aliens could sling at Earth an asteroid just big enough to completely devastate the planetary surface. The means by which those possessed of the technology to traverse the stars could destroy human life are simply multitudinous.

Rule Number Two: Any alien species technologically capable of travelling from one star system to another would have no possible logical rationale for imprisoning, enslaving, or destroying mankind.
Interstellar travelers must possess not only immense destructive capacity, but constructive, as well. It is probable that any beings capable of such a feat are able to build robots who can serve them far more effectively (and obediently) that enslaved humans. And yes, even if they are robotic, they can still build robots. They would be able to derive organic nutrients and water from sources other than the plunder of inhabited planets. And, beyond all of this, space travel is a cooperative and intellectual endeavor -- one which flourishes in a society which lauds the intellect, the characteristic which breeds philosophical thinking and realization of the futility of violence. And so, any beings with the power to travel from one star system to another are likely to shrug off as absurd the notion of attacking a planet peopled by beings barely able to send on object to the edge of their own star system.

We really would be better off fearing an invasion of orcs.