My cousin, Brendon, and his girlfriend, Jenna, were in town this weekend. Both of them read my blog and Jenna told me I'm clearly knowledgeable about what I'm talking about, but still wrong. That response in itself deserves a post, which I may or may not get around to. Still, it would be fun to deviate a bit away from controversial topics and I thought of alien invasion. If aliens invaded, what would be their motivation?
Natural resources is a favorite Hollywood staple. Aliens will destroy our planet to take our stuff, likely minerals (Independence Day comes to mind but the physical land might be valuable, too (in The Day the Earth Stood Still, alien invaders attempted to terraformed our planet).
This is quite possibly the least likely reason aliens would go through the trouble of global conquest. The universe is filled with minerals and other resources that no one's using and are much easier to get to than stuff on an occupied planet. Maybe other extraterrestrials make harvesting Vega asteroids difficult, but even in the boundaries of our solar system there's still quite a lot to draw on.
Unless aliens are after biomass (i.e. living things). Perhaps there's some technology or feeding requirement that requires biomass (or they just want us as slaves), and Earth is the best place to get it. Lots of biomass with little ability to defend it. In that case, the best strategy probably wouldn't be a messy mass attack on the resource you're trying to harvest. Aliens would likely be much more insidious with probably a pathogen--killing everyone or knocking us all out while leaving the bodies around for collection later.
Racism is a likely motivation. This pops up in a lot of Sci-Fi television shows (Star Trek, Stargate, Babylon 5). A similar train of thought is a concern for humanity and a fear of what will happen if we're allowed to continue to live. Given many humans mistakenly fall into one of these two categories, I'd like it's a reasonable motivation for aliens to kill us.
Technology is a justification that's rarely explored. This is understandable but not for the reasons you probably think. Most assume that if aliens know how to travel faster than light, they are clearly more advanced than us on all other levels. That is far from certain, especially since an alien brain will probably not be wired the same way as a human one. As Richard Dawkins suggests, it is entirely possible there's a group of extraterrestrial life who intuitively understand the theory of relativity but have difficulties grasping principles of air resistance and other "slow" speed phenomena.
But it would be rather clear that the easiest way to get technology is to trade for it, not to take it by force. If such aliens were interested in our technology, they would more likely be peaceful. The one exception to this rule is if they discover it's easier to kill us than to learn a single human language. That would certainly be a sad state of affairs for us.
Then there's the possibility that aliens need to build a intergalactic superhighway along a path that intersects with Earth's orbit. That's a bit unlikely given how big space is ("Really big," according to Douglas Adams) but it could still be true. And if galactic eminent domain is as potent as our terrestrial version, then considered us conquered.