You might also want to check out "First Contact : X-Corps". It was an RPG Kickstarter I backed years ago. Never played it in the end but it covers defense by Earth against an alien invasion.
Ah, I think I got a pdf of that one - inspired by the X-COM series of PC games. Yeah, I might take another look at that one.
I'm sure someone mentioned Living Steel a while back, which was an alien invasion based RPG too? But I seem to remember it was insanely crunchy rules and possibly done by the same company that published an ALIENS game in the '90s (?)