I started creating this campaign world many years ago. I forget what inspired it, I think it was initially a series of novels I had read, and the titles or author of which I have long since forgotten. It didn’t really feature the Earth being invaded, but aliens did come visit, and they recruited humans to fight an enemy that was threating them, and would eventually threaten Earth as well. It featured the concept that all the alien races were at a disadvantage, but that Humans were uniquely gifted at combat.
So I started tinkering with an idea for a role playing setting. Then I read the The Freedom Series by Anne McCaffrey. Humans (and other aliens) abducted from their home worlds by another race, and used as colonists, to break a planet in.
There was a short lived sci-fi TV series a number of years ago, Dark Skies I think it was called. It contributed some thoughts to the melting pot as well.
And so my idea evolved, and the movie Independence Day came out, and again I revisited some of the premises of my idea.
The biggest influence though was the Worldwar alternative history series by Harry Turtledove. Where aliens invade during the height of World War II.
And so taking ideas here and there, I constructed what I envisioned as a trilogy of campaign adventures set in this world of my making. Sadly I never did even complete the first installment, but what I did finish will be posted here.