Bear with me here.
Let's assume that a civilization or three has developed the technology to explore interstellar space using sunlight drives of one type or another (the actual type doesn't matter too much at this point). Let us additionally state that Einstein was absolutely right and there is no way for a physical object to achieve light speed - it remains an actual barrier.
Now, let's assume that for various reasons somebody gets mad at somebody else, decides they're a threat to their species survival, or otherwise just decides that somebody else needs their ass kicked.
Let's assume that these somebodies are all loose connections of a home world and a dozen or so colonies and that evryone's been expanding and developing for about the same length of time. Nobody knows where the other guy's home world is or where all the enemy colonies are.
Now, let's start a war and fight it logically and scientifically.
What's the first move such civilizations might make in the first step of an interstellar war?
(mods, since this is speculation with supposedly a foundation in science feel free to move it at will to the sf&f firm, please)