Click to viewIf there's one thing that science fiction has taught us, it's that it's almost a certainty that Earth will be invaded by an alien force at some point in our lifetime. Luckily, we've also learned that humanity will most likely forget our differences, pull together and defeat said alien force through unity, the resourcefulness of an under-appreciated member of society and potentially the common cold, but that doesn't change the fact that - as GI Joe would tell us - knowing is half the battle. So we present to you a guide to the different types of alien invasion, so that you can know whether to join the fight or betray us all to your potential new alien overlords.Openly Aggressive Alien Invaders You know the type; they come with their guns and their bombs, to quote Dolores O'Riordan, and just try to blow shit up until we surrender. We've seen their kind in War Of The Worlds (whichever version you want), any Dalek-themed episode of Doctor Who, Independence Day or even Transformers, and they're the kind of alien invaders that the world expects - Straight, to the point, and despite their superior firepower, ultimately prepared to fall to our superior intellect, pluck and whatever contagious diseases may be going around at the time. Passively Aggressive Alien Invaders These aliens are much sneakier and more difficult to deal with. Oh, they come with promises of all manner of intergalactic treats and peaceful transactions, but behind their oddly perfect grins, they're really lizard people who eat mice whole. I speak, of course, of the aliens from V, but we've seen similar tactics from Secret Invasion's Skrulls ("Embrace change," indeed. The only change they want for us is the one from breathing to six feet under) and, memorably, Mars Attacks's big-brained psuedo-pacifists: Secret Invaders Where the Skrulls went wrong, of course, was coming out of the alien closet. If they'd just stuck to their original plan of being undetectably undercover, then world domination could've been theirs much more easily - if less profitably for Marvel Comics. Think of how successful the 1970s Invasion of The Body Snatchers was (and try to ignore Nicole Kidman's shameful The Invasion, while you're at it), or the fact that poor David Vincent never quite managed to completely save the day in The Invaders. Particularly popular in times of American foreign affair paranoia, secret invaders can also generally be identified by the nearby presence of a title that includes some variation on the word "invasion." Accidental Invaders It's not just The Day The Earth Stood Still's Klaatu who found himself mistaken for a hostile force; poor ET almost got himself dissected by an overzealous government with cell-phone handguns, as well, and don't get me started on those poor musical bastards from Close Encounters Of The Third Kind. Call it the human disease: seeing something different and just expecting the worst without provocation. I mean, sometimes, even those who do intend to cause destruction aren't really entirely bad guys: think about the Vogons in The Hitchhiker's Guide To The Galaxy, who destroy the Earth only because it's in the way, or Star Trek: The Next Generation's Borg, who only want us all to be brothers and sisters and share the same interests, likes and dislikes, and hive mind. Is it that wrong to condemn them? Well, yes. After all, if we don't condemn them before they destroy our planet, how are we supposed to do it afterwards? If there's another thing that science fiction has taught us, after all, it's that whichever aliens are going to make contact with the human race, there's a very high percentage that their language's version of the words "widespread destruction and/or subjugation of the native population" are going to appear somewhere on their list of things to do. It may seem cruel and overly suspicious, but in this dog-obliterates-dog-with-raygun universe, the case can always be made that it's worth cutting open a couple of friendly aliens who just want to phone home if it means that we don't end up under the extra-terrestrial boot heel of some inhuman Space Hitler. Take my word for it, people: hope for the best all you want, but if you happen to make first contact with an alien, make sure that you do so with at least a knife handly, just in case. And whatever you do, remember: Don't cover your mouth when you sneeze.