Mere interplanetary invasion - attack from another world of our own Solar System - is hard to plot plausibly, given modern knowledge of the unlikelihood (in fact the virtual impossibility) that any other world circling our Sun could harbour a technological civilization which might pose a threat to us. But what about invasion from another star system?
Here the problem is different. It's plausible enough to imagine that starfaring species exist out there, and that one of them might want to grab Earth for their own use. But if so, how could we possibly make a fight of it against a race advanced enough to have conquered interstellar space? Wouldn't it be so one-sided as to furnish scant material for a story? The tale, it seems, would be one of destruction rather than invasion. Rather like a human army versus an anthill.
One possible answer might be that a race from a dying planet, wishing to change worlds, might be so few in number that our vastly greater numbers might compensate for our equally vast technological inferiority. This is perhaps the rationale for the old TV series The Invaders, starring Roy Thinnes.
Another possible answer is that the foe might be essentially parasitic, with no need for its own civilization; it merely infiltrates the host culture and turns it against itself. This is what happens in R A Heinlein's classic The Puppet Masters (which I count as a tale of interstellar invasion, as although the immediate origin of the invaders in this novel is Titan, the suggestion is that their ultimate origin is interstellar).
A perhaps even more ingenious solution to the problem of plausibility can be found in Larry Niven and Jerry Pournelle's novel Footfall, and also in Stephen King's The Tommyknockers. The stories are very different - in Footfall the invasion affects the whole world, in The Tommyknockers it is local in impact and is defeated by the efforts of one man. But in both of these novels what gives us a chance against the invaders is the fact that they are in a sense "freeloaders" or scavengers, who have stolen or inherited a spacefaring technology which they could never have invented for themselves.
They are nevertheless formidable opponents, and the reader can feel a proper awe at the immensity of the task of defeating them. But at least the idea of resistance is not utterly ridiculous, as it would be if we were fighting the actual originators of the starships.
All the examples I have given also deal effectively with another possible objection - namely, the objection that races advanced enough to master star travel would also be advanced enough to refrain from imperialism. I'm not sure that the argument is conclusive in any case, but certainly it need not apply if the users of the starships are a desperate few, or parasites, or the galactic equivalent of delinquent joyriders.
