Trope-a-Day (R): Alien Invasion

Alien Invasion: Happens a lot less often than you might think.  Seriously, planetary invasion across interstellar distances is Really Damn Hard (the number of units in the field you need to control and occupy a planet populated by billions alone is pretty astronomical, even if they are all self-replicating war machines).  There’s no point in doing it for the resources, since if you have even a quarter of the tech you need to mount said invasion, your space infrastructure means you’re already swimming in resources.  It’s mostly the preserve of Scary Dogmatists and the Interstellar League of Tribal Chiefdoms, and no-one takes those people seriously, particularly because their backward socioforms tend to hinder them from being serious players.

Which isn’t to say that the prospect of one of those terrifying postsophont powers wafting in a half-kilo of replicant neurovirus and pwning all your brains doesn’t keep strategic defense planners up late at night, but so far, those guys haven’t shown any particular urge to earn an entry in the Top Ten Galactic Atrocities List.