You make the assumption people only go to war for 'rational' reasons.
When one of our own terrestrial non-rational nation-states/cultures shows signs of creating a manned space program that can go even as far as the moon, I'll consider the point.
Reducing any debate about aliens really comes down to the Fermi Paradox. Namely: "Where are they?"
If the Drake Equation's variables in the Milky way are such that 2-3 intelligent tool-using species exist at any given time out of (what's the latest count? 400 Billion stars?), I'd think that over billions of years, the odds are in favor of at least one alien species having "made it". And with exponential growth or even replicating technology like Von Neumann probes that build copies from local materials, one such species should have covered the galaxy by now with some sort of indication of their presence.
So this leaves you with a few possible outcomes.
- Intelligent tool using life is
really rare. So rare that they don't show up even on cosmological timescales. Humanity is alone or at least, effectively so.
- They have covered the galaxy, we just haven't seen the evidence of it yet. Perhaps even misinterpreting some of the evidence as "natural phenomena".
- Something else is at work. Something happens to advanced species that survive that steers them into paradigms of existence we have not considered, or do not yet have the tools to consider.
Or you don't consider their intelligence to be equal.
Even then, there are other reasons besides "better safe than sorry" including:
1. They have planet you want (if you want to expand population and don't like living in orbital habs)
2. They have resources you want (note, actinides/lanthanides are found more readily on planets than asteroids)
3. It's less energy intensive/easier to harvest complex chemicals found naturally than synthesize (in some cases) even if you have access to CHON from other places--even if you can master interstellar flight (I'm thinking ark ship, not FTL, so energy costs can dominate
4. You want a better zoo on your home world, and the humans started fighting when you borrowed a few
1. Terraforming/alienforming is easier/more economical against the time and energy expenditure of interstellar travel. Especially in your slow-ark scenario that makes energy costs dominate.
2. I think that even if higher heavy metal/transuranics concentrations are in terrestrial worlds, there'll always be one that's more convenient (statistically speaking) that you can just KEW then mine it like asteroids. So even if they do attack/mine Earth, it's still just a scenario where they blow us up, rather than "invade" in a way that can be fought.
3. Since all the chemical reactions we know of found in nature are either powered by the energy percentage of sunlight intercepted by Earth's surface, or the geothermal energy of mid-oceanic ridges. I guess I could see sampling missions for compounds you hadn't been able to imagine, but I find it unlikely that such an advanced species couldn't imagine/simulate most any possible chemistry through advanced computing and evolutionary algorithms, etc. And the energy requirements of interstellar travel, even in slow-arks, start running up against Earth's entire solar energy influx, or our geothermal output.
4. I see this as the one remotely possible scenario that you've raised. However, I'd see it more as a sample collection mission before KEW'ing Earth on my evil game-theory hypothesis of "better safe than sorry".
* The better safe than sorry extermination theory has it's own problems though. Going around the Galaxy exterminating other species always runs the risk an even more advanced species is watching, and now you've given them REALLY good reason to wipe you out.
So, it's possible that if the "better safe than sorry" paradigm does dominate, then it may just mean everybody hides, because statistically speaking, stamping out other races is even more dangerous than existing and being detected through technological activity.
