I've put some thought into this subject for my science fiction universe (MTU?) 

Two assumptions I've made (now the argument will start) about my universe.  1) There are already humans "out there" when Earth's population starts to send out colony ships. Human kind was seeded throughout space by my equivalent of "Grandfather" 2) The same attitudes, prejudices and ideologies that Earth's people had when they left Earth staid with them. 

As a result I have several "empires" for lack of better term that have formed.  There were also several that preexisted Earth's Diaspora. The two that there will be quite a few stories wrapped around is a totalitarian society The Democratic Socialist Republic or Aligned Systems.  (Think USSR on steroids)

The other is The United Federation of Free Planets.  (Think a combination pf  British Empire/USA) 

The two could not be of starker contrast.  The DPRAS was formed as a result of a few ambitious people who wanted to share their socialist vision with all of known space causing the First Ideology War.   UFFP formed as a result of a need for mutual defense against DPRAS aggression, but note this is a federal style of government not a confederation.  (There is a third empire that is a confederation, but I haven't done all the development on it yet.) 

While DPRAS continues to attempt to conquer more of known space (although the Second Ideology War caused sufficient losses in ships and personnel they've been quiet) the UFFP allows worlds to join if they petition for membership. 

UFFP encourages free trade both within UFFP space and outside UFFP space DPRAS restricts trade between the outside and itself only through trading outposts.  Goods are severely taxed and very few see their way to the common folks by DPRAS. 

The UFFP is more like 3I in that individual worlds are pretty much allowed to do their own thing without interference from the Federal Government unless issues that conflict with the Articles of Membership arise.  For instance, having a separate treaty with DPRAS would be considered a "no-no" and probably get the planetary government "spanked." 

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