Its been more than a century since HG Wells introduced the world to the modern concept of the alien invasion. Yet, with all the invasion movies, books and TV shows that have come out since, its been rather frustrating that few writers/creators have been able to come up with a credible reason for aliens to actually invade.
The problem seems to be that usually the motivation is based largely on historical precedent in human history. The thing is that the technological hurdles necessary to cross vast interstellar distances, if overcome, would have no precedent in our history.
For instance...
Slaves: In the past, slaves were a perfectly valid reason for invaders from "advanced" civilizations to venture out into someone else's territory seize their lands, and then press some portion of the native population into servitude. After all, those "advanced" civilizations were dependent on large workforces to make their economies and societies function. Civilizations were built thanks to brute strength and access to muscle and man power. However, as has become increasingly obvious in most post industrial societies...the more advanced your technology gets, the fewer people you actually need to do the work. We have not even reached the level of an interstellar civilization and already we have the problem that we have more people than jobs. Who needs slaves when you can build machines to do most of the labor?
Procreation: This is possibly the least likely justification for an alien invasion. The idea that an alien species would turn to another alien species in order to stay viable is simply idiotic. It would be like humans turning to slugs to solve our fertility problems. At least human and slugs have common genetic material.
Water: Its everywhere...why start a costly war over a commonly available material?
Food: This is at least theoretically possible...or is it? I can't travel to certain parts of the world without some common delacacies making me sick. Would an alien with a totally different biology be able to consume ANY biological material from Earth safely?
Minerals: It seems to me that any minerals found on Earth could be just as easily found somewhere else without having to deal with crabby inhabitants that likely would not take kindly to you taking their stuff.
So really...can anyone come up with a good reason for aliens to invade?
