In space, everyone knows where everyone is (scattering sensors around is cheap, and the second law of thermodynamics means there's no way to hide unless you know which direction your enemy's looking from), and everyone can kill everyone else quite easily. Just the velocities involved mean an interstellar ship is a missile (and probably one that can devastate planetary ecosystems); there is no defense other than to strike first, no way to meaningfully armour a ship (even counter-missiles are impractical due to conservation of momentum), and honestly, when it comes to war, no reason to even have ships at all - just launch missiles directly from whatever your bases are.
If we survive long enough as a race to have interstellar travel, I predict people living on artificial habitats inside gas giants and/or stars, the location/flight-plan of each a closely guarded secret (and probably communicating/trading only by meeting on neutral ground), since that seems like the only place you could hide. If you're out in the open, any idiot with a grudge can wipe out your civilization.