Duck, be reminded of the resourcefulness and hardiness of humans. During the civil war, weapons and vehicles that were never developed elsewhere were created, and after the Civil War, all the US had was growth, growth, growth. 

And while there may be so many worlds, I don't think it would be plausible for them to all be united under a single banner, government, or alliance. There are always opposing groups; while it may be likely they'll say they're part of a particular treaty, I don't think they'll be united. On at least a dozen worlds in a thousand, there would be a different social system, whether socialist, communist, fascist, etc.
There is no way to keep hold of that many planets, for anyone. Where there are creatures that have opinions and ideas that oppose someone else's, there will be division, and there will be separation. The only reason my nation is united is because they're all half-sentient, dreamy and slow to respond, mainly controlled as a hivemind type of entity.

Summary: I don't think it reasonable that all conflicts would be between nation/group vs. nation/group. I think the majority of conflicts would be just keeping the entire nation in one entity, rather than a fracture or something.