That's kind of an ignorant view. Considering the amount of resources in the uninhabited parts of the universe (which is beyond a staggering amount) why would any one enter conflict over a small planet (us).
Any species capable of interstellar travel is going to be able to pull resources out of pure energy. They don't mine, or need our water. They don't care what we do, except maybe they observe us and snicker.
From "The Killing Star"
When we put our heads together and tried to list everything we could say with certainty about other civilizations, without having actually met them, all that we knew boiled down to three simple laws of alien behavior:
THEIR SURVIVAL WILL BE MORE IMPORTANT THAN OUR SURVIVAL.
If an alien species has to choose between them and us, they won't choose us. It is difficult to imagine a contrary case; species don't survive by being self-sacrificing.
WIMPS DON'T BECOME TOP DOGS.
No species makes it to the top by being passive. The species in charge of any given planet will be highly intelligent, alert, aggressive, and ruthless when necessary.
THEY WILL ASSUME THAT THE FIRST TWO LAWS APPLY TO US.
That's just why they would be willing... but it gets worse: There's an imperative.
Once a certain amount of technology and capacity and know-how has fled the homeworld: we, as a species, become capable of attacking another species even if our homeworld is wiped out. In short: there's a limited window during which species A could reliably exterminate species B without worrying that some missed portion of species B could retaliate.
So imagine some alien sees us right now. If they killed everyone on Earth (perhaps bombarded us with relativistic weapons), we could neither mount a defense nor seek retribution. 200 years from now: perhaps there would be enough off-planet resources that, after the destruction of Earth we could secretly build relativistic weapons of our own and retaliate.
So the only way for that species to guarantee its own survival is to wipe out potential rivals early.