Horselords

Horselords, by David Cook, is the first book in The Empires trilogy. "Between the valiant kingdoms of the western Realms and the exotic eastern lands of Kara-Tur lies a vast, unexplored kingdom. For centuries, the "civilized" peoples of the Forgotten Realms have given little notice to these barbarians. Now, a powerful leader has united the wild horsemen into a mighty force–an army powerful enough to challenge the world."

Summary
For centuries, the civilized people of the eastern and western realms have dismissed the nomadic inhabitants of the vast middle land as barbarians, but now these nomads are rising with the might to challenge the world.