You cant win a war in the sky, you got to have troops to hold the ground.
Assuming you want to hold the ground. If for some reason you are simply interested in wiping out vermin (the alien race) who pose a threat to you, perhaps in the way that a nest of very nasty hornets in a tree right outside your back door would, you could simply sterilize the planet by nuking the entire surface from orbit (or even further away) . . .
-- Ronn! :)
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