The Will of the Great Kingdom to Win
Finally, the Oeridians created their great empire because their great commanders, mages, and tribal leaders believed that this was their destiny. Driven from their homelands by a great cataclysm, they founded the great capital of Rauxes nearly 4,000 miles from their ancient homeland. No other people traveled nearly as far of their own volition (the Suel were forced to do so).
Restless and driven, great pre-Aerdy commanders of warfare such as Andorann, Leuk-O, with his massive magical juggernaut, and Tuerny the Merciless conquered vast swathes of land because this was what they had to do. No matter how rich and fertile any particular land might be, there was always an imperative to expand further, to head beyond, to conquer the vastness of the Flanaess and gain the longed-for glory of triumph and rulership.
Other races did not possess this burning fire. The Flan were a simple people; nomads do not have such territorial imperatives, and they did not have the technology nor the desire to build great castles and fortifications to defend the lands they originally held. The Bakluni were too given to contemplation, mysticism, and hampered by a fatalism which the reverence of Istus brings. The Suel were simply too cruel, too mean of heart and spirit. Oeridian soldiers were fully prepared to die for the greatness of their people. The Suel were too selfish, too limited in their world view, to die for anything. And so, the Oeridians triumphed.
Carl Sargent. Greyhawk Adventures, Ivid the Undying, 1995