How about the Clans in Battletech?

Israel might be a useful model. It has mandatory military service for all citizens regardless of gender with limited exceptions for religious beliefs and medical issues, and unlike most modern societies with universal conscription, it engages in wars, military exercises, etc. frequently enough to be a relevant example. Now, Israel is currently a representative democracy, so it's not totally what you're looking for, but it might serve as a starting point,

Lots of good ideas about what the world might be like -- one thought: we don't have many examples of people *growing up* in completely artificial environments; I think *that* might make at least as big an impression as a war.

I think that child-care on a space ship would be hugely communal (for reasons of efficient utilization of resources) and the primary family unit might be subordinate in key ways to the "class" or other group social structure (this might also be the case if there were a lot of orphans).

I wanted to chime in on "how to make sure the players get it:"

This is a technique thing that I've thought a lot about. Here's what I've come up with:

1) You need to have the world written up. No one is going to read a novel (unless you write *very* well), so the document should be structured with clear sections, and bite-sized, bullet-point chunks that would.

- Give the players the basic, common-knowledge their PC's would have
- List key NPC's (say 3-7 people) with basic information like name, gender, age, race (if there are aliens) and role and no more than one or two short sentences of description covering both demeanor & physical appearance
- Introduce them to KEY elements of their perspective, particularly where it's different from 20th/21st century people in your player's country
- Give a bit of flavor about the world through things like terminology, names, and other references
- Use graphics as much as possible -- they're fundamentally interesting and, done well, can convey a *LOT* of information about the world and the culture (so inset pictures of the fleet ships, maps of known-space, etc.) I would get everything from Google because I'm not an artist or anything)
- Give short snippets of first-person narrative, interviews with key NPC's, or purport to be excerpts from official documents

2) You also need to talk (briefly) through the document so during / before character creation and during the game not actually assume anyone has read / remembered it. Reference it during the game.

I've both done and been given these kinds of things and they rock. With the right formatting they're highly skimmable; players pull terms and ideas and references out of them during game to give a remarkable sense of solidity to the world.

I also find that skill in putting this kind of thing together is key -- the best one's I've seen are funny. This stuff is dry, inherently.

Here's an example: I was doing a game set at about 2012 (5 years from now) for characters who would be about 20 at the time. Although these characters would be contemporaries of the actually players (mostly in their... eh... well, ah... old), I thought that playing someone *20 years old* in 2012 would be significantly alien to our group that I gave them the following write up as the first section of a document I'm describing:

I include it since folks read it and it got a decent reaction (mainly "Lord, I'm OLD!" but during the game people have been pretty good about making era-appropriate references and expressing curiosity when older characters make 80's and 90's era references)

   Class of 2010
•   You were born in 1992
•   The year before you were born the Soviet Union dissolved. The Berlin Wall fell two years earlier. You never knew the Evil Empire
•   America had been at war – briefly – the year before – with Iraq
•   Your first memories might be around 1995 – You were too young to see Toy Story in the theaters that year; your parents might have watched the aftermath of the Oklahoma City Bombing on TV
•   Or they might have watched the OJ Simpson trial.
•   You were five in 1997 when Titanic became the highest grossing film of all time – too young to see it, or appreciate its intense and transient impact on world culture (by the time you were 12, and old enough for Leonardo DiCaprio to be on your radar he was playing eccentric millionaire Howard Hughes in The Aviator (2004). Kate Winslet had disappeared into movies you've never heard of like Romance and Cigarettes)
•   By the time you were eight you were probably aware that everyone would soon be rich (something called the Dow broke 10,000 in March 1999. The next month two teenagers killed 12 of their fellow students and a teacher in what remains the worst school shooting on record) and then the world would end on December 31st, 2000 A.D.
•   9/11 would have been nearly incomprehensible from a geopolitical standpoint. Nothing in your experience would have given you a framework or context for the events of that day. At nine years old the events of that day seem less like a political act and more like a violent, arbitrary act of nature comparable to Hurricane Katrina four years later. The death tolls are almost the same (2500 for the storm, 3000 for the terrorism)
•   In 2002 you're 10 years old and just at the edge American Idol becomes a pop-culture phenomenon and remains there as you age into the lower end of its demographic.
•   The techno fantasy of enduring ascendency and Dow 30,000 of just three years earlier seem like Narnia-level fantasy. The bubble is gone. The country seems to be in a haze of danger and a build up to war (isn't there a war already going on?)
•   The year after that the Iraq War starts (no one you know calls it "Gulf War II"). You're 11 and perhaps old enough to realize we're fighting a war in someplace called Afghanistan as well.
•   You entered High School in 2005, when you turned 14. President Bush embarks on his second. It does not begin auspiciously (the Hurricane). Signs are not good – if anything you hear that things may get worse.
•   But if the weather, the world, and dad's job seemed a little uncertain, at least the old homestead was doing well. Housing prices were at an all time high
•    In 2007 the subprime housing crisis morphed into the global credit crunch. On the eve of your sweet 16 (2008) people would be talking about a collapse comparable to the Great Depression. The optimists said it would only be "as bad as the 70's." You know people – middle class people – who have lost their jobs (Bear Stearns). You know people – middle class people – who have lost their house

Cheers,
-E.

Have you read the Amtrak Wars? The depiction of the Federation is pretty close to what you describe - the society is geared for war against the mutants who have inheirited the world after a nuclear war. Its very regimented but also quite human in its foibles. The main thrust of the book is about how individuals from such a society cope when they meet their enemy and ultimately fall for them and the conflicts that produces.  

If you weren't up for an that type story you could perhaps emphasise the social obligation to respond to orders. Everyone expects it, even civilians, and to rebel is call into question the war and the survival of the species.

Civilian characters who appear fit and able to serve could receive a certain social penalty as people wonder why they aren't serving in the military. This has real life parrallels in WW1 and WW2 where able non-combatants were imprisoned or ostracised. Perhaps the characters are operating for a government agency that is so secret they can't tell anyone. In WW2, the code breakers in Bletchley were forbidden from talking about their work to even their families, not just in the war but afterwards and for same reasons could not join the military. One C.O in Bletchley actually went as far as writing to the children of some of the code breakers, now grown up, once the documents had been declassified to explain how critical their work had been to the war effort.

I suppose you want to answer - what sort of military do they have? A broadly meritocratic one or a semi-feudal one where the high positions are appointed from a limited range of people? That will in turn shape the society that emerges.