The Yophenthean Empire expanded mightily during the centuries of 300 to 600 AI. This article lists and outlines the causes of the expansion.

Chronology

492 AI to 513 AI Zelukian War Yophenthea wars against Midretasso and her allies; Dagdos Gargastin supreme military commander and admiral of Midretasso, campaigns to defeat Erechóreb
513 AI Battle of Siul concluds the Zelukian War

Sources of Yophenthean Cultural and Military Domination

superior organizational practices
highly centralized government based on cult of the sun emperor.
Use of allies and client states to defray cost of conquest and government
Religious inculcation of regional aristocracy into the sun cult.
speedy communications
Superior military intelligence due to sun priest seership
use of provincial levies to amplify legions (giants, centaurs, Moigthe soldiers, dwarven mercenaries, merfolk naval forces, Ithatian navy)
use of demi-mortal powers: sunburst, negation of magical powers of enemy wizards and sorcerers, detection of lies, charismatic presence,
benefits to subjects include: elimination pirates and brigands, uniform law code, advanced agricultural techniques reduce or eliminate famine,