: First we must make one of two assumptions. One, that the
: war was fought by some western noble or king who
: wanted to break away from the Cath'Bruig Empire. Or
: two, the West had already broken with the eastern
: Empire and the war was fought only within the
: political bounds of the West.
I'd go for option two. For one thing, the info on the Twenty Years War says that it was fought in and around Covenant. Covenant isn't strategically placed if you're trying to fight off a Cath Bruig army whose only means of reaching the Province is by World Knot or through the Cloudspine.
The war is referred to as a war of succession. Since it was fought in Covenant's immediate vicinity, I would imagine the succession in question was of who was to rule Covenant. This could indeed be an issue of the Province's candidate versus the Cath Bruig candidate (or puppet, perhaps? but that's another issue), however this would not restrict the battle to Covenant.
: A) The political fragmentation of the most powerful
: Empire in the world.
Indeed, if the Province splintered from the Empire it would constitute political fragmentation, but I seriously doubt that a relatively minor schism between the two would result in either one being severely militarily weakened. If the fir'bolg could figure out that they couldn't survive on their own, then it's a sure bet that the Empire and the Province helped each other out, no matter what their relations. Surely those relations may have had an impact upon how big of a response was provided by either side, but
I think the advent of the fallen lords made many of these "fragmentations" easily forgotten.
: B) The weakening of that Empire. A twenty-year civil war
: would cost a lot, both in gold and men.
True, although here I nitpick. The Hundred-Years' War, for example, wasn't one big war (although it was no less messy). If there really was twenty years of war in Covenant alone, it would easily deplete the population. This would achieve quite a lot in Balor's favor, but it's only one city, and I have a hard time believing that they fought for twenty years nonstop. It might have been twenty years of constant revolution, one king being replaced by another, or a guerilla war, or a series of conflicts spread over twenty years' total.
