Logistics of the Great Kingdom
Just as magic proved a major element for defeating the Suel folk in particular, the superior “technology” of the Oeridians helped them defeat the Bakluni and Flan. More familiar with long marches and extended campaigns of warfare, the Oeridians knew the vital need for supplies, building bridges, gathering information on natural obstacles, fording places at rivers, and very simple elements of warfare such as the value of archers and how and when to use them. This was a key element in the defeat of the Bakluni and Suel people, who were at the time behind the Oeridian development of crossbows and chain mail armor.
A notable special case are the magical roads, the dirawaen in Old Oeridian, crafted by combat mages using earth elementals they otherwise commanded in battle. With special weather-resistant magics woven into their crafting, these roads made travel and provisioning much easier for the Oeridian invaders. The Oeridians were just one step ahead with such simple elements of warfare. Add to this the work of the priests of Zilchus and the recipe for Oeridian success reveals itself.
Carl Sargent. Greyhawk Adventures, Ivid the Undying, 1995