Halfway through my response to this, I realized that someone had already written it, and in a much more entertaining fashion. So I'll just point you to his post instead:

Three Worlds Collide, by Yudkowsky

http://lesswrong.com/lw/y5/the_babyeating_aliens_18/

"The most controversial story I’ve ever written. Starts with the baby-eating aliens and moves on from there."