So I spent a lot of my Christmas Eve afternoon thumbing through the Wikipedia pages on Fermi's Paradox (if there are so many stars and planets out there that could host intelligent life, why haven't we found any yet?) and then thinking about the implications of one particular answer. If it is possible for a civilization to possess a means of accelerating objects to 90% of the speed of light or better (henceforth known as relativistic weaponry), it's quite possible that everyone is hiding out of fear of getting shot first.
You see the problem. If Alien Species AAA is purely pragmatic and interested in survival, then their best option is to hit us as soon as they learn we exist, because they have to assume we're going to do the same to them if we get the chance. That's the premise of The Killing Star (spoiler alert!) and it's one that I'm trying to find reasons to work around.*
It seems to me that there are basically three strategies for a species in this situation:
A) Kill other civilizations as soon as they appear. Also, disperse your own civilization throughout as many planets, asteroids, interstellar spacecraft, star systems, etc. as you can, in the hopes that any civilization that does find you won't get all of you.
B) Stay quiet and hope that nobody finds you. Also, disperse.
C) Attempt to communicate with other civilizations and set up an interstellar truce/balance of terror. Especially, disperse.
The Atomic Rockets page discounts strategy C out of hand ("they're aliens! There's no way you can figure out how to communicate with them!") and shoves A and B into a prisoner's dilemma game that attempts to prove the validity of A. While communication would be a huge problem, there are at least common concepts (i.e. the atomic weight of hydrogen, the speed of light, a list of prime numbers à la Contact, some other constant that would hopefully be common in spacefaring civilizations) that we might be able to establish and work from there. Ideally, you only need to be able to transmit the concept of a dead man's switch.
In my opinion, if you're not going to attack alien civilizations offhand, the next best option is to let them know that you have weapons aimed at their home planet and won't hesitate to fire if you see their projectiles coming. (If something has already been fired and is on its way, well, the electromagnetic wave carrying your warning will ironically cross theirs in mid-space, arriving just enough ahead of their weapons for their leaders to read it and say "Haha! Oops!") I'm a fan of firing warning shots outside their solar system, although there's no guaranteed way to show that it is a warning shot and not a genuine miss or act of war. Perhaps by accelerating a transmitter carrying your message instead of a simple boulder? It's not perfect, but it probably beats the first two...
Option B, hiding and dispersing, essentially hands your species' fate over to Species AAA (for there's no guarantee that they won't find everybody on your various colonies). You may think that staying silent guarantees safety, since even an insanely aggressive Species AAA wouldn't shoot at every planet they could find (it's a big universe). However, that's not guaranteed. This entire scenario only becomes reality if relativistic weaponry is technically possible to build and fire effectively, and it's a safe bet that any Species BBB would've figured out how to exploit the electromagnetic spectrum well before that. Earth is surrounded by an expanding shell of radio and TV transmissions even now, which (at least theoretically) reveals us to anyone that's watching. I'm guessing that most species would have something similar, which puts BBB in the same position that Earth is in in The Killing Star (it's not a good place to be).
So that's not really an option. But Species AAA is in an even worse spot if it wantonly kills Species BBB. If you miss any shots, you're doomed, since the enemy will have seen you miss and can fire back at leisure before you ever know you've missed (the lightspeed limit is a bear sometimes). To pile on top of that, what happens if Species AAA shoots at Species BBB and Species C, D and E all see it? Species AAA probably has its own radio-transmission shell, making them visible, and firing off relativistic projectiles would only attract extra attention (and identify them as a serious threat to whoever saw them). It's even possible that Species PPP has decided to act as a policeman in a certain area of the galaxy, enforcing a no-relativistic-weapons ban among species under its purview. The penalty, of course, would be wiping out the offending planet.
So what have we established? Namely, that in a world of perfect relativistic killing weapons, there is really no good option to pursue. What you do depends on how paranoid your decision-making class is, how expensive your weapon is to build and fire (in terms of construction and energy used) and how good your communications skills are. Also, a healthy amount of dumb luck.
The good news is that this scenario-of mutually assured interstellar destruction-isn't guaranteed to come to pass. One of the Atomic Rockets users pointed out that relativistic weapons are far from the only method of conducting interstellar war, even in "hard" sci-fi scenarios where you're stuck with this universe's physical laws. And aiming these weapons would be insanely difficult: you would probably be firing across scores of light-years, finding yourself having to account for the motion of the enemy planet (s) in orbit, the motion of their solar system and your solar system, plus the effects of any masses (stars, brown dwarfs, etc.) along the way that might deform your missile's path. Even a tiny course-correction might cause the aggressor species to miss entirely. Such complications, plus the difficulty and expense of constructing a mass accelerator capable of reaching out and touching someone across vast distances, could help prevent this strategy from ever being used.
*(A lot of this stuff is touched upon in the Atomic Rockets page, and I'm trying not to repeat them. Nonetheless, while I didn't lift words or sentences directly from AR, credit for the inspiration of this post and a lot of ideas in it go to them.)