I thought they said they were being invaded by the Colonial Defense Force? I don't see no aliens.
I caught that too.
Although.. to play devil's advocate... if you were an alien race, and you had time and good infiltration, what better way to bring humanity to its knees than so stage an attack indistinguishable from one by said Colonial Defense Force?
The two sides grind each other until one is weak; then you steam roll that side, while keeping the formerly winning side on the defensive, and then, after you've established a base of operations in the (I'm assuming Colonial Defense Force will be the first to knuckle under here) CDF space, you finish off Earth.
Divide, Conquer, Profit!
I'd say of course CDF + Earth would realize and unite, but, hey, this sets up the sequel: Call of Duty: Infinite Guerrilla Warfare XIV!!!!