I have talked before about what kind of weapons that ships in space would use, but I have not yet talked about the real weapons of war. Destroying your enemies military directly is only one way to win a war, and as a rule is not very efficient.
You only win a war under two conditions, the enemy can not resist your military any longer or they give up.
There are several options to do this for two space faring enemies on different planets.
The first way would be to shove a large asteroid at your target, if you can make a battleship you can do this. We could do it today if we wanted to kill ourselves in the most expensive way possible.
The fact that this would be so easy would introduce a cold war element to any potential conflict between planetary enemies. You could even have the war be very asymmetrical, where one side has a tenth the military resources of the other but are not conquered solely because they could take their large opponent with them.
Defending against this would be a massive undertaking, much more massive them attacking would be. This is often one reason in science fiction for attacking, its cheaper by far to take out your twenty nearest neighbors then to constantly be ready to defend against any of them. And the technology to attack is much easier to make then defend.
And of course it just gets harder to defender yourself if your talking about two enemies in different systems, defending against an asteroid going half the speed of light would be a tricky thing to say the least. It would probably involve putting a whole lot of debris in the way and hoping for the best.
Such an asteroid could easily kill billions of people in less then hour. However that is only if the people are on a planet. While most habitats would probably be soft targets they would have the advantage of being small and spread out. You could do this simply so it would be hard to kill you all off, and it would not be possible to do so all at once like on a planet.
The vulnerabilities of planets and habitats are such that many people think that hiding would be the best strategy, in fact it might be why we have yet to detect aliens.
Personally I doubt this would really work for several reasons. First it all the effort you put into hiding would be wasted the second your found out. I would rather have a warship or two then the knowledge that I was hidden for twenty years. Second it would be really hard to hide, the inferred radiation that any body with humans in it would give you away, as would any use of radio.