The galaxy houses many different civilisations spread out over billions of planets. Most of these societies are unaware of each other, being that the physical distance between them is incredibly large. There are some, though, who evolve relatively close together and are able to, eventually, form multi-species, multi-planet coalitions.
One particular coalition lasted for half a million years before it splintered and fell into something that wasn't quite war, but certainly wasn't peace.
During this phase of the coalition, there were three main groups which had coalesced out of the collapse. Largely, there was no open conflict between any of them, however, as tends to happen, a number of smaller factions had also formed and most of these were either small military dictatorships or roving groups of bandits and pirates. There was open conflict between these smaller groups and the larger factions, however, since the larger factions controlled 99% of the coalitions infrastructure, the smaller groups couldn't really make any ground on them. So, stalemate. More or less.
Many plans were formed to bring the factions back into alignment and reform the coalition, however the root cause of the collapse was never really addressed and was always the elephant in the room during these discussions.
The issue was energy.
There were a lot of people in the coalition and they all needed a constant supply of power. Most of the individual societies in the coalition had power sources that would happily fulfil the needs of their citizens, however, there was one civilisation who had designed and built a super efficient, super cheap power station. It was, essentially, a series of solar panels that were launched into orbit and were converting sunlight into electricity non-stop.
They had announced right before the collapse – which wasn't a coincidence – that theses panels provided clean unlimited energy to their entire population and that it was all a pilot program for a bigger venture which was to encircle an entire planet in them, and then an entire star system and keep building out like that.
Others in the coalition thought this unfair and demanded the technology and/or access to the panels themselves. When this was refused – based on the idea that these panels were still experimental – political talks ground to a halt and the entire coalition teetered on the brink of collapse.
And it would have collapsed entirely if it wasn't for a convergence of events which left the plans for the panels in the public sphere and a number of people involved in keeping them a secret dead or humiliated. And as such, while the coalition did collapse as a result of all these events, it wasn't as messy or as permanent as it could have been.
The largest, however not the most powerful, of the post-coalition factions had been trying to build some of these panels for a long time, however they didn't have access to a sufficient quantity of a required chemical and were busy trying to both research replacements and talk their way to a supply with a faction which did have access to the needed supplies.
Both of these paths were long and difficult for the faction to complete and eventually, they both merged into one, that is to say, the diplomatic talks ended up being the foundations for the new coalition to be built on.
Of course, it never works the way its supposed to and while two of the larger factions merged, it was an imbalanced event which, instead of a universal embrace, caused the remaining factions to push away which led to paranoia and open conflicts.
The problem was the newly merged mega faction, while only 48% of the old coalition, controlled nearly 65% of the military power. The remaining factions were concerned as there were a lot of people high up in the mega-faction who had publicly suggested, with a straight face, that reforming the coalition with overwhelming military force was the way to go. With people like that in decision making positions within the new faction, paranoia and hostility, even towards their own members, took hold in the remaining factions. The larger one closed all its borders, preventing anyone from coming or going and started a program of conscription which very quickly sent them into a cold civil war.
The smaller, more nomadic factions didn't really lose much, as they didn't have anything to begin with, however, they too ran the risk of facing the incredibly superior military power of the mega-faction if they pushed at them too hard, so they effectively allied themselves with the remaining large faction and, basically, waited to see what would happen.
What did happen was … Nothing.
The mega-faction effectively sat and built themselves up from the inside as a galactic congress, admitting new members from new civilisations who could reach them and governing surprisingly well. Outside the closed off borders of the mega-faction, though, this was all considered propaganda or written off as a conspiracy theory, despite how true it actually was.
The larger of the remaining ex-coalition factions also built themselves up slowly, worried about the mega-faction, but still, at least on paper, willing to talk about merging again to reform the coalition. However, the congress wasn't listening and fobbed them off at each turn.
It was the mini-factions which had the really interesting story. In the years following the mega-factions creation, these nomadic, opportunistic groups found themselves allied with each other, diametrically opposed to each other, allied with the congress against the other factions, allied with the other factions against the congress and almost every other variant thereof. Some of them were able to merge with the congress, others merged with the other factions. Some merged with each other, then split, then re-merged. An entirely new one was created when separatists broke off from both the congress and the ex-coalition faction and merged with each other. This new faction, which began swallowing the other mini-factions very quickly became a threat to the stable existence of the current status quo, and quite suddenly and not at all coincidentally, talk of reforming the coalition was back on the table.