If this were a superhero comic book, the Martians would be the super villains, hatching a scheme to invade the earth. They are active antagonists – they set the plot in motion. They're the ones who upset a perfectly nice thing. But really, we know the Martians are antagonists because they use a Heat-Ray on a town.
Interestingly, by the end of the book Martians, when the Martians are wailing and dying, there's a chance that we might empathize with them a wee bit. The Martians only invaded Earth because all of their resources on Mars are gone. Isn't that kind of sad? The narrator even wonders if, one day, humans might start our own war of the worlds with Venus.