This debate has been going on for decades but on the grand scheme of things what would happen? Well the fact is if this civilization is more advanced then us then most likely we would see the same thing that happened between Christopher Columbus and the Native Americans but on a much larger scale. (See: Christopher Columbus: The Untold Story ) There is no plausible way that these aliens would have conquered greed. A lot of scientist say that we should not seek out alien interaction just for these purposes. A lot of people say that if they are so advanced that they traveled all the way here then they must be a peaceful race and all they want to do is further extended their knowledge along with ours about the universe. That however is a very optimistic view. At first that is probably what they would say their intentions are but who's to say they would not have a much more sinister plan in mind? On another note if we found a alien planet that had a less advanced society on it what do you think we would do? Yeah history does seem to repeat itself. So if we do ever find another life form that is alien then we should treed lightly because we never know what's afoot.
I'm sure that if aliens visited our planet they wouldn't be here just to consume it's resources, unless they were so advanced their aged planet could not provide for them anymore. We are not all that advanced as a planet and we are more interested in learning about other worldly life than just destroying it.
Maybe they've seen so many humans that they wouldn't even be interested in visiting us? Maybe they would see us as a cancer and just vaporize us? What ever they do, I just hope it's something good.
If extraterrestrial life forms were to come to this planet, they would probably do so on a frequency humans aren't able to detect, operating under senses we don't have. If there was some sort of physical interaction, I imagine they'd treat humans like humans treat creatures less intelligent than ourselves. Put them in cages, metaphorically speaking.