Don't get me started on the aliens in Harry Turtledove's WorldWar alt history. Being completely genre-naïve, they cross interstellar space and then proceed to f

Don't get me started on the aliens in Harry Turtledove's WorldWar alt history. Being completely genre-naïve, they cross interstellar space and then proceed to fight WWII-era earthlings with approx F22-level technology and not unsurprisingly get bogged down in a stupid attritional war. Why didn't they just drop rocks on us from orbit?