All that a "war" would consist of is aliens taking one of those giant asteroids in the Oort Cloud, painting it matte black, and placing it on a collision course with the Earth. Without light reflecting off of it, we won't be able to see it unless we're looking directly at it and see that it's blocking out stars. Ka-boom. No more humanity.
If they want the planet intact, all they have to do is use a smaller asteroid and some sort of dispersal mechanism for whatever the top ten fastest-acting, deadliest, and cruelest genetically engineered diseases possible are. That way, humanity is simultaneously suffering from a global disease epidemic in the wake of a cataclysmic meteor strike (hundred-foot waves, etc.) that would leave tens of millions of refugees wandering and unaccounted for while death-bringing virii rain down from above.
In any case, I doubt any "war" would involve soldiers. Except maybe robotic ones. Ships, sure, probably robotic ones that can secretly mine asteroids and build more of themselves. I doubt any aliens of the biological sort are likely to put themselves in harm's way. So this wouldn't be a World War II guns-blazing affair with battles and stuff.
Of course any of this could be us attacking them, but the principles remain the same.