From the Brief History:
“When the Invoked Devastation came upon the Baklunish, their own magi brought down the Rain of Colorless fire in a terrible curse” p. 5From description of the Dry Steppes:
“Once the area of was well-watered and fertile... but it was destroyed by the Invoked Devastation in the war with the Suloise.” p. 21
From the description of the Sea of Dust:
“In return for a terrible magical attack, the Suloise lands were inundated by a nearly invisible fiery rain...” p. 26And that is all that the Gazetteer has to say on the matter.
What's clear is that a not-insignificant portion of the Baklunish lands were devastated by a terrible curse, and that the Bakkies* let loose an even more thorough and complete devastation on the Suliemen in the form of the Rain of Colorless Fire. But nowhere is it definitively stated that it was actually the Sulies who Invoked the Devastation that started the Great Cataclysm Race, leaving open the possibility that a third party was responsible for The Big D. But who?
*Is it racist to call them Bakkies?
Well, how about the Oeridians; they were also victims of Bakluni expansion, the turmoil of the war having forced them to flee their lands and head east into the Flannaes. As there is no mention of the Oeridian homelands in the gaz, could it be that any evidence of their native lands was wiped out by the Devastation? Perhaps Oeridian mages left a curse upon their fertile homeland which was triggered by the departure of the last Oeridian peoples. And, not wanting to incite a rain of colorless fire on their own asses, they decided to keep mum on the topic.
Or was it the Flann? They too suffered heavily from Bakluni expansionism as their once peaceful lands were stampeded by refugees of Oeridian and Suloise stock alike. Perhaps Flannish devastationists teamed up with dwarven demolitions experts, gnomish telemarketers, and elven poets to unleash the Devastation in hopes of stemming the flow.
Or even the Bakluni: The text from the Brief History in particular seems to lend itself to the interpretation that the Devastation was something that the Bakluni may have brought on themselves. Perhaps while developing their apocalyptic technology they accidentally unleashed the Devastation on their own lands. Undeterred, they sharpened up their game and let fly the Rain on their enemies. Or maybe it wasn't an accident; maybe they went all Guernica on some minority population in their realm--like the Oeridans--and then took it a step further, blamed it on the Suel, and used it as provocation for unleashing the Rain del Fuego sin Color, sort of like how Poland provoked Nazi Germany into starting WWII.
Or, indeed, it might have been the Suloise all along. The cataloguers of the Gazetteer may have been restrained in ascribing the Devastation to the Suloise because the RoCF was so thorough and widespread in its destruction that no Suloise who were in-the-know about the Big D survived to take credit for it.
Gygax, E. Gary. World of Greyhawk Gazetteer, 1980. TSR Hobbies, Inc., Lake Geneva, WI