For those who aren't as close to the "inside baseball" of Greyhawk as I am, that means that I am waffling between using the setting as shown in the folio and Gold Box set, and the setting as it is shown in The Adventure Begins and the Living Greyhawk Gazetteer. The one is before the "Greyhawk Wars" (Gods how I loathe that term) and the other is after.
Part of the reason is that I want to use the D&D Next rules for this campaign, and something about using the latest version of the rules leads me to want to use the latest version of the setting. Too, I find there is a great connection between the 1E rules (or Adventures Dark and Deep) and pre-Wars Greyhawk in my own mind, and I want to give the new game a spin; first with the playtest rules, and then with the final version once they're published in August.
I know the post-Wars Greyhawk has a bad reputation in a lot of quarters, but I don't particularly dislike it. I certainly don't find it "unrealistic" as some of my fellow Greyhawk fans do; there were periods in European history that make the border-shuffling and kingdom destruction/building of that 15 year stretch in the Flanaess seem positively sedate.
Tonally, it is a very different setting, however. In 576, evil is present, but not omnipresent. Strong bastions of Good such as Keoland, Nyrond, the Iron League, and Furyondy (and their allies) keep forces such as Iuz, the Great Kingdom, and organized humanoids and giants at bay. There are skirmishes, but large-scale warfare is rare, and the civilized lands (even those of evil bent) are, as a rule, prosperous and peaceful on a large scale. It feels "sunny, with clouds in the distance."
In 591, none of this is true. Not only has Iuz conquered the north-central Flanaess, but humanoids taken the Pomarj and southern Wild Coast, the Iron League is shattered and many of its members conquered either by the remnants of Aerdy and the Scarlet Brotherhood, Geoff is overrun, Almor is destroyed, etc. The scars of warfare are everywhere; cities lie in ruins all across the continent (especially in the now-riven Great Kingdom), there is famine, and trade is much lighter than it was. Too, the three greatest Good realms are severely weakened and are themselves in perilous positions. It feels "cloudy, with smoke from burning villages making my eyes water."
I'd make 591-601 a period of exhausted rebuilding and increasingly tense stand-offs. Give Furyondy-Veluna a chance to rebuild its northern frontiers and come to terms with the presence of the Empire of Iuz. Let Nyrond and the North Kingdom fence over the former lands of Almor, which are finally starting once more to blossom. Have North Kingdom and Ahlissa establish a border, with Rauxes still a no-man's-land where most folk fear to tread. Irongate and Free Onnwal, with support from the Kingdom of Sunndi, expel the last of the Scarlet Brotherhood troops from Onnwal and retake Scant.
I wouldn't roll back everything, of course; the Empire of the Pomarj would still be a growing threat, creeping up the Wild Coast after their abortive attempt to invade Celene and increasing pressure from the Principality of Ulek. Iuz would loom over everything, of course, but the empire is fragile in the extreme, ready to burst into a dozen shards if something were to happen to the Old One. After all, it happened once at the hands of Zagyg, and almost happened again at the hands of the False Iggwilv. Third time's a charm?