"An Outside Context Problem was the sort of thing most civilizations encountered just once, and which they tended to encounter rather in the same way a sentence encountered a full stop."
- – Iain M. Banks, summing up the end result of Star Trek type societies like The Interex encountering the Imperium quite nicely
"All we wanted was to be left alone."
- – Last words of a Xeno captain member of the Diasporex (another Star Trek type society that had the misfortune of encountering the Imperium during the Great Crusade) as he lay dying and his civilization is purged
The Interex (both the demonym and the empire, used like "the Imperium") were basically what the Tau think they are. The word "Interex" derives from the real life word "Interrex": the title of the leader in the ancient Roman Kingdom that takes control during an Interregnum, with the title literally meaning "Between Kings". Considering their eventual fate it's just as unsubtle a name as Ferrus fucking Manus.
The Interex, probably the closest thing we can glimpse about Human Civilization during the period of enlightenment known as the Dark Age of Technology (or at least a positive example of a newly isolated colony adapting in the aftermath of the DAOT's collapse), was made up of humans working and living alongside the alien race called the Kinebrach, which were like space gorillas. It is notable to mention that the Interex learned about Chaos from, and were tutored by, the Eldar, likely coming into contact with Craftworlds making their early getaway, and who had seen first hand what could happen or even just some random Craftworld making its slow galactic cruise, as they would still be functioning in their intended role as trading vehicles at the time, it could have even been the Exodites; the Kinebrach are also said to have warned their new human partners about the dangerous nature of the Warp. The Interex keep records of Humanity's early voyages into the void, telling tales of humans making treaties with demonic powers, with some even becoming daemons themselves. During first contact with the Kinebrach the human colonialists fought with their future partners, with the Kinebrach's technology proving to be even more advanced then that used by the human forces at the time. The conflict wouldn't last long before the two sides came to a mutually beneficial arrangement; proving the Imperiums blind dogma that all Xenos are just wicked monsters worthy of nothing but death wrong; their union would last for three thousand years. The forces of the Great Crusade would stumble upon warning beacons stationed around a feral world, warning visitors to leave the planet along as it was essentially a prison for a very deadly arachnid like species; much like a wild life preserve. The Legions, always eager to claim another world, didn't even bother to work out what the warnings meant and charged head first into the jungles, which ended up costing them dearly. After a while other ships arrived in orbit and wanted to know what the hell they were playing at, and why they didn't listen to the warnings that they had left. The Interex ships that arrived were bigger and more advanced than those of the Imperium, and, after some exasperated eye rolls, they invited the bloodied Imperial forces back to their place to get patched up. Unlike the Imperium, the Interex had no interest in any form of expansionism, and were quite content with their small territory. They were aware of and fought against Chaos (which they called Kaos), being tutored by the Eldar during their travels, unlike the Imperium at the time which officially denied the existence of daemons and spirits; the interex had been suspicious of the imperial forces, seeing their aggressive and war like behaviour as potential Chaos corruption, however when they discovered that the legions were completely ignorant, they were horrified that they had been traveling the stars without any sort of protection or knowledge. The Interex language was augmented/accompanied by some kind of music based on universal math called the Aria, which made them cool and not prissy. Their warriors could ride on four-legged robots to form a centaur-like warrior that fought with laser arrows that could easily penetrate power armour. Regular Interex warriors could just about take an Astartes in single combat, mostly because the Astartes underestimated them and because the Interex's fighting styles were almost unrecognizable to them. They felt that war was not the solution to all life's problems, and this idealism would lead to their demise, as after finally taking the Interex seriously, the Astartes quickly kicked their asses. For all the tech it lacks, the Imperium still possessed outstanding material science - as a result, bolters and chainswords were and are really dangerous, even to the Interex.
In the first Horus Heresy book, Horus meets them and is curious about the idea of cooperation with other species and decides to actually negotiate with them even though he knows the Interex makes use of robots and AI, which are abolished by the Emperor (thanks to that whole Men of Iron thing that fucked over everyone before), because he feels that the galaxy is too grimdark and the Imperium purges things too often, much to his legion and other imperial forces present fury, who angrily protested that such a thing went against the Emperors words to not suffer the Xenos to live. Horus told them to shut the hell up and that now that he was warmaster they will look into the option of not instantly killing every Xenos that crosses their path. When it seems that everything is going well, and the Imperium might just not suck if Horus thinks like this, Erebus steals a Kinebrach sword called an Anathame, THE MURDER SWORD (that would be later wielded by a champion of Nurgle much later on to fatally wound Horus; Fuck Erebus) from a war museum called the Hall of Devices, blows the museum up, and starts a war. Horus is scratched and sulks off to feel bad while the Interex are mentioned to have been purged in an offhand mention in the next book.
The Interex makes an excellent contrast against the Imperium in several ways. Firstly, there's the comparison between them (or what little we know of them) and the Imperium both pre-Heresy (already showing the rot beneath its gilt facade) and post-Heresy (techno-barbaric theocratic fascist feudalistic hellhole). The Interex made the truth about the denizens of the Warp plain and simple to all its people, ensuring every member of their culture knew exactly what the Chaos Gods were after and the dangers they posed. The result of this education? Chaos basically couldn't get its hooks into any Interex: they were so powerless against them they had to manipulate the Space Marine Legions into doing their dirty work for them, though, it's more likely that they were left alive solely for the plan to corrupt Horus, especially since Chaos has proven multiple times that it can appear wherever the Hell (pun!) it pleases and unleash its legions of daemons. The Interex would have been purged as a threat (can't let the Anathema get his hands on their Warp-tech, after all) if not for their usefulness in tainting the Warmaster.
A legit point to argue is that their lack of militarization would have led to catastrophe if the Tyranids or Necrons were to show up on their doorstep, or even one of the Ork Empires of the time (as one of them required the efforts of multiple legions to topple, whereas the Interex were wiped out by a
single legion single expeditionary fleet of a single legion). The Interex clearly had the ability to defend themselves against smaller groups of hostile xenos (as shown by their defeat of the Megarachnids) but just weren't geared to fight a galaxy-conquering force. It is still intriguing to imagine what might have happened if Erebus' plan was foiled, or what other Legions might have made of the Interex if they'd come across them before Luna Wolves did - like, say, the idealistic pragmatic Ultramarines, or the technophilic Iron Hands. Or even better, the Thousand Sons. That would have been hilarious (from a safe distance, anyway).
But were the Interex truly immune to Chaos?
Let us consider for a moment a few points: first off the Interex had received help from other alien races with some experience on dealing with Chaos; indeed, this is an advantage which may not have been wielded by many human civilizations in the setting.
Secondly there is the simple fact that the Interex was a post-scarcity economy; this alone means that next to no one would like to attempt to deal with enemy forces out of physical desperation, which is often one of the major reasons why Chaos takes hold in the poverty-stricken worlds of the Imperium. Like in real life, the vast majority of people will not feel any interest in joining the enemy or a strange cult when things go well.
Thirdly, the Interex was a collection of small territories when compared with the Imperium, so it was far easier for them to manage their population and bring education to the masses while keeping a better control over them, and if this sounds familiar to you, great, you are paying attention.
Finally, Chaos really didn't need to corrupt anyone to take hold over the Interex - all it needed was to use someone to wipe them out on their behalf. It also might have been possible for Chaos to corrupt some of them eventually. As Radical Inquisitors have shown, being fully educated on what Chaos is and its dangers doesn't prevent you from falling to it, and in some cases it actually helps the fall. In the Interex's case, their awareness of Chaos led them into a state of complacency that allowed the Ruinous Powers to destroy them with the help of outside agents.
Ultimately it can be recognized that the Interex looked like a possible path for a better future for mankind, but all it takes to fuck it up is one guy willing to take advantage of a bad decision like choosing to keep and study a Daemon Weapon (or any Chaos artifact really) instead of destroying it. So, they could be seen as a case in point that the Emperor's way is the right way since it isn't vulnerable to one asstard fucking everything up. Oh, sure, one asstard can try to and do a lot of damage, but ten thousand years and a Great Rift later, the Imperium is still kicking ass whereas the Interex are extinct.
The kinebrach were a race of space gorillas that the human colonialists of the DAOT fought and then assimilated. They were technology advanced (more so then the humans at the time) and were civilized and cooperative enough to not be nuked back to the stone age like the megarachnids. Fortunately, the two sides were sane enough to realize that aliens can actually work together to achieve great things.
The Human colonialists and Kinebrach effectively merged their civilizations, becoming what is now known as the Interex. The humans were clearly in a leadership position when they encountered the Legions and were prepared to help out the Imperial forces even though they had their growing suspicions.
The Kinebrach had clearly dealt with the Warp in the past, and were clearly skilled in its use in creating weapons. The Anathame is also called the Kinebrach sword because it was made by the Kinebrach at some point in their history, as the sword stolen from the museum had a magical disease on it, that when keyed to a target would course 100% certain death to that individual; its shards are also capable of cutting a rift in both time and space. The wound caused by this weapon to Horus would lead to his sons seeking out the Chaos cult that would end up corrupting him entirely.