The idea that an advanced alien civilization would invade us for resources is pretty ridiculous, and clearly the result of years of bad sci fi.
First of all, strictly speaking about raw elements here, we got nothing you can't find a lot more of elsewhere - and without having to resort to mass genocide of a sentient race to get it. All elements originate from star's fusion ovens - spread across the universe by them going nova. Doubt we got anything really special or unique to just this planet.
You lot were mentioning water - hell just in this system there's other places you can go where you can easily get as much as you like. Saturn's rings are largely composed of ice asteroids. We're talking metric tons per piece here. In addition, there's Europa, Mars, and who knows how many small planetoids/KBOs that might have it. Pretty sure you can get whatever you need, in large quantities, elsewhere. Invading a one in a million (billion?) small planet that happens to be home to a sentient race you need to wipe out first just to get small amounts of resources you can get elsewhere wouldn't be practical.
And the very idea such a race advanced enough to even reach us is short on water or has to resort to interstellar wars to get their resources is ridiculous, anyone seriously considering this is failing to grasp just how big our little corner of the galaxy is, let alone the whole galaxy/universe.
Besides, a race that advanced may just be able to reorganize matter into whatever forms they need. At any rate, take bad sci fi for what it is - good passing entertainment. It has no place in any serious discussion on what relations with a real alien race would be like.