[quote=Brovo]planets don't run out of ammo
[/quote]

Oh, sure they do. How many rockets exist on Earth which are capable of reaching a high-earth orbit? I don't have a number for you, but long story short it's $Finite. I sort of went over some of the advantages of attacking a planet from space in my first post -- these things matter. Could somebody just build more? I mean, yeah, you COULD -- but wouldn't the productive advantage go to the militarized, industrial-type capitals, and not to the spacefaring settler types? And they wouldn't even have to build as much stuff. Your delivery system, for an orbiting ship striking a planet, might weigh 1000 pounds, while the delivery system for planet-to-space would have to be in excess of -- well, [url=http://coolcosmos.ipac.caltech.edu/ask/268-How-much-did-the-Space-Shuttle-weigh-]an awful lot[/url]. Physics favor the aggressors, in a big and meaningful way. That's perfect imperial stomping grounds. 

On the communications/light barrier business -- look, I don't know how (or if) someone's gonna overcome that, I'm just saying, Ghengis Khan managed, Alexander managed, Caesar took Gaul, you know..... shit happens, great people can accomplish great things even if there are obstacles. If it was easy, everyone would be doing it.