Unfortunately most of these questions are incredibly difficult to answer (which might explain why they're generally ignored) and require a great deal of assumption on our part. Any conceivable (yet realistic) reason we can come up with to explain why an alien race would come to earth is limited by logic (duh). While at first that might seems a bit strange consider this. In 2009 a study was published that determined cats weren't able to solve the same types of logic puzzle that dogs (and humans) could. While some (myself included) decided to use this piece of evidence to settle the eternal war between cats and dogs, Britta Osthaus the psychologist who performed the study came to a much more interesting conclusion; cats think differently not because they are less intelligent but because they have a different system of logic.
Even assuming that Miss Osthaus' is wrong about logic though, we are inherently limited to our own flawed system of logic and our own anthropic bias. Even our system of mathematics, something that I always assumed to be near perfect, has been proven by Godel's Incompleteness Theorem to have inherent problems. What's worse is the only way to truly remove an anthropic bias is to (and I hate saying this cause it sounds like a Disney movie) believe anything is possible.
In other words, any kind of planning we do regarding an alien invasion is bound to our own system of logic and while it's highly unlikely the aliens have the same reasoning we do, I think it's a bit more fun (and terrifying) to pretend they do, if for no other reason than to play-through childish fantasies.
So starting at the beginning; Why would an alien civilization invade earth (assuming of course they aren't just doing it for fun and they aren't killing us because of some slight we've made against them)? Well logically they would do it for any of the basic reasons we humans use to justify invasions. Basically this boils down to three things they could be seeking:
1)They could be seeking to make use of the Earth's Resources. (which I don't actually think is valid)
2)They could be seeking to colonize planets suitable to their species.
3)They could be seeking to liberate us from a perceived oppressor or to educate us.
I guess this means in the next couple posts I'll expand upon these reasons and (if somebody would like to help) debate the validity of these reasons or any additional ones in the comments section.
Rusty is the co-founder of How To Survive Alien Invasion Novels, and gets up to all kinds of shenanigans planning for any and all kinds of apocalypse when he's not busy reading, writing, or yo-yoing. Keep up with him and Rachel on Facebook and Twitter to get cool, space related news or click here to read more of his thoughts on the terror of alien invasion novels.