One of the guys from my D&D group sent me this. It’s a collection of ideas, quotes, tables, definitions, and examples of interstellar empires.
His timing could not have been better, as I’ve just recently finished reading several of LeGuin’s Hainish novels.
One of the greatest difficulties in maintaining an Interstellar Empire is, of course, distance. Distances in space are unfathomably, impossibly great. Consider that when Voyager took the famous Tiny Blue Dot photo of earth, it was 1.7 billion miles from earth and still not in interstellar space.
Any empire is going to be limited by its ability to react to situations on its fringes, and, even with light-speed travel, the fringes of an interstellar empire might be several decades away from main worlds. In the Hainish books, which largely take place on fringe and backwater worlds, the Federation of Worlds often appears distant, useless and incompetent, because the significant distances between worlds means that any actions taken involve such great lag times that it is an impractical body whose main impact on its member planets is exaction of taxes for a brewing galactic war (which it loses).
Even with instantaneous communication across space, being able to send response forces to deal with any sort of conflict situation is nearly impossible.
Interstellar political bodies are therefore problematic. There are difficulties in governance, difficulties in enforcing laws and difficulties in asset protection. How do they come about, then, if they are so impractical? Eventually, perhaps, technology would exist to shorten the temporal distances between worlds, but at what point would that make a federation practical?
Probably the hardest science fiction work I’ve read was Kim Stanley Robinson’s Mars trilogy, which chronicles the initial colonization of Mars, the process of its terraformation, and resulting political crises of a whole new world that is beyond the effective governing control of earth. Even the relatively short distance between Earth and Mars results in vastly divergent cultural ideas, identities, and political solutions to the practical problems which they face. Isolation, in effect, means independence.
In an interstellar setting, what one would most likely find is a series of tributary worlds, whose status as tributaries is the result of an initial large show of force. Eventually, the tributaries, which, unless left under the control of some autocrat in the name of an empire, would realize that it was more or less independent, test its independence, and cease becoming a tributary, at which point the empire would decide whether or not it was worth the time and resource investment to re-establish its tributary status. And even under an imperial autocrat, the world might become independent under the autocrat who realizes that he can keep the world’s wealth to himself and the empire will only challenge him if he is particularly egregious in his defiance of imperial will.
Anyway, you’ll find more and better ideas than I can articulate here in the article I linked.