Although Cherryh was active in the 1970s, I think her 1981 Downbelow Station was my first exposure to her work in general and to her Alliance-Union setting in specific. Not that this is strictly speaking an Alliance-Union novel ... at least not until towards the end.
I remember finding it a bit of a slog at the time. Clearly other readers disagreed with me, because it not only won the 1982 Hugo Award for Best Novel, but was named by Locus as one of the top fifty SF novels of all time.
The good news for humanity is that by the 24th century, humans have spread far beyond the confines of the solar system, first at sub-light speeds and later with FTL. The bad news is that the human worlds outside the solar system are caught in a vast interstellar war, with the predatory Earth Company on one side, the authoritarian slave-drivers of the Union on the other, and a handful of neutrals, mainly merchants and a few stations, caught in the middle.
The war between Company and Union has dragged on and on, far beyond the point the balance sheets would justify. Earth Company is ready to beg Union for peace. The problem is, the Earth Company's Fleet is not ready to stand down, and it answers not to the Company now, but to Commander Conrad Mazian.
One of the early events that drove a wedge between Earth Company and its colonies was the discovery of a life-bearing world humans called Pell (and later Downbelow). By the time of the novel, other life-bearing worlds have been discovered—chief among them Cyteen, capital of the Union—and Downbelow is something of a backwater. Until now the war has largely bypassed the station and the world it orbits.
The first sign that this is going to change comes in the form of a vast wave of refugees, survivors of Union's push to snap up neutral stations and their systems. Downbelow Station is huge by our standards, but tiny compared to a world; not only can its life support systems be overloaded but there's always the risk of riot or worse.
Meanwhile, the Company's attempts to negotiate a peace with Union encounter difficulties. Union believes that its victory is assured; it has very little reason to deal equitably with its former masters or with anyone else who gets in its way. Downbelow Station, for example, would make a fine addition to Union. If the ruling Konstantin family doesn't see it that way, Union would be happy to arrange as many assassinations as it takes to place a ruler beholden to Union in charge.
Mazian, for his part, is down to a small handful of ships. Without resupply, the war of attrition can only end in his annihilation. At least, the war with Union can only end that way.
A war with isolationist, ill-prepared Earth, on the other hand....
I suspect that the reason I found this such a slog in 1981 is because, rather astoundingly for someone who had ten novels under her belt before writing this one, Cherryh starts off Downbelow Station with an interminable infodump about the future history leading up to the Company War. It's an inexplicable choice. Much of it is irrelevant to the events of the novel; the information that is relevant could have been doled out to the reader in other ways—and not as a giant, plot-killing block of text.
Not only does the infodump start the story by slamming on the brakes, but it also draws attention to details of Cherryh's worldbuilding which, on second look, seem less than plausible. It's never clear what it was that was valuable enough to justify building space stations (one per system, apparently) in nine lifeless systems. Once I start paying attention to details like that, they just mount up; for example, Pell disrupted interstellar trade because it could supply agricultural goods cheaper than Earth could, which means someone somewhere was making money by shipping food between stars. At sublight speeds.
Also, it's pretty clear from the population levels given for individual stations and the number of systems referenced that Union would have a hard time scraping up the population of a decent city in all its systems combined, despite which they don't have any trouble wielding power equal to that of Earth Company .
I did like the detail that many Earthlike worlds may actually be fairly hostile to Earth life. Pell has an ecosystem as complex as Earth's but it is not Earth Two; humans need technological assistance to stay alive on the surface.
Still, dodgy worldbuilding is hardly unique to Downbelow Station. For me, the major downer in the book is the relentlessly unpleasantness of the setting. Most people living off Earth are merchants barely subsisting on the brink of bad luck and sudden death. They are subjects of a grey, joyless oligarchy (if they are lucky enough to live on or near Pell) or one of a horde of azis, brainwashed slaves, (if they are not lucky enough to live on or near Pell). The political grouping that becomes Alliance isn't much fun, but holy crap, Union is much much worse. And ... Alliance's freedom depends on cutting a deal with Signy Mallory, who is at best a useful monster given to sexually exploiting prisoners. (A deal must be cut, because she is the captain of the Norway, one of the few remaining Earth Fleet warships.)
It's an interesting decision to make Union a slave-holding society, given that a lot of readers will see parallels between Union's desire to rid itself of Earth Company and the attempt of a certain cabal of tax-dodging, slave-using colonials to gain independence in the 18th century. There have been many SF works that riffed on that particular independence movement, but very few (IIRC) have paid much attention to its peculiar institution.
(1989's Cyteen suggests that the architects of the azi system intended it as a short term solution to their labour shortage, but 1980's Serpent’s Reach makes it clear that azis remained in use for a long, long time. Really, the universe would have been so much better off had the founders of Union ended up with their heads displayed on spikes.)
[Editor’s note: James, why don’t you tell us how you really feel?]
While the setting is grim, many of the characters are sympathetic. That helps less than you might expect.
Downbelow Station was published when the average SF book was still shorter than the norm today. Its 432 pages made it stand out against the backdrop of more slender books. In fact, the page count is a little misleading, because, as part of the effort to contain soaring book prices in the age of stagflation, publishers like DAW crammed as many words to the page as possible ; this is a longer novel than mere page count would indicate. Later English language editions have had as many as 526 pages. Aside from the unfortunate infodump, none of this is wasted space; there's a lot going on in this book and Cherryh needs all of those pages.
I cannot say I love this book but others do, which is why it won a Hugo and why it has been in print for more than thirty years . Downbelow Station is available from DAW.
1: Another way to look at it is that to the people of Earth, all Union and all of what becomes Alliance is so laughably underpopulated and irrelevant that it can be safely ignored by anyone living on Earth, the home of the vast majority of the species.
There's an alternate version of this novel where Mazian manages to reach Earth with his pitiful fleet, only to discover the difference between being able to threaten a polity of billions and being able to rule it.
2: I began rereading my MMPB months ago, but the font is so small and my eyesight so bad I had to settle for reading a few pages a day and stopping when the eyestrain kicked in.
3: Amazon and B&N appear to think the paperback is temporarily unavailable, NEW, in paper (used copies abound). However, an ebook version is readily available.