The Greyhawk Wars were a series of esclating conflicts involving dozens of nations in the Flanaess begining in 569 CY. The Wars devoured armies and resources without mercy and resulted in one of the largest losses of life in Oerth's history. The cost and damage done to every nation in the Flanaess would not be suprpassed until the Drow Invasion.
Several countries were wiped from the map, and others shattered into hundreds of petty holdings. The fighting raged for two years, and only ended with the historic Pact of Greyhawk in 572 CY.