A decade after the original Greyhawk boxed set, the boys at TSR decided to shake up the setting with a continental war that served as the backstory for the ambitious (and perhaps ill-advised) Greyhawk Wars game. In an era during which the company didn't seem to care what happened to the setting, they fortuitously handed over the bulk of Greyhawk's output to British writer Carl Sargent.
The boxed set is fiendishly well written, completely changing the tone of the campaign setting while remaining surprisingly loyal to the source material. In the hands of a lesser designer (like nearly TSR's entire staff at the time), From the Ashes would have been a disaster. Instead, it's a masterpiece.
Sargent was a shining star at TSR's creative nadir, but sales on the Greyhawk line and thus managerial interest had slumped to the point that almost no one noticed. Long-suffering Greyhawk fans, after enduring crap like Puppets, Childsplay, and Gargoyles, had largely wandered away.
Despite the lack of a spotlight (or perhaps because of it), Sargent's work brought a depth to the campaign setting it had previously lacked, bringing a noticably darker tone to the world. Sargent's Greyhawk is grim place just emerging from years of continental warfare.
Many of the DM "trump cards" left by Gygax in 1983's World of Greyhawk Fantasy Game Setting had been played during the Greyhawk Wars, effectively ruining wide swaths of the world. The development caused a rift in Greyhawk fandom coinciding with the rise of D&D fandom on the Internet, and thus the question of whether one "accepted" the events of the Greyhawk Wars and From the Ashes divided Greyhawk fandom along its most fundamental axis.
Several years later, From the Ashes remains a primary text in the canon of the Greyhawk campaign setting and a must-have for any serious student of D&D.