Oyo Empire metropolitan Oyo. This may have been fine in the 15th century when Oyo controlled only its heartland. But to make and maintain farther conquest, the structure underwent several changes. Oyo maintained a semi-standing army of specialist cavalry soldiers called the "Eso", the "Esho" or, formally, the Eso of Ikoyi. These were 70 junior war chiefs who were nominated by the Oyo Mesi and confirmed by the Alaafin of Oyo. The Eso were appointed for their military skill without regard to heritage and were led by the Are-Ona-Kakanfo. After Oyo's return from exile, the post of Are-Ona-Kakanfo was established as the
West Africa all-powerful Akan Empire of Ashanti, while Ife rose to prominence around the 14th century. Further east, Oyo arose as the dominant Yoruba state and the Aro Confederacy as a dominant Igbo state in modern-day Nigeria. The Kingdom of Nri was a West African medieval state in the present-day southeastern Nigeria and a subgroup of the Igbo people. The Kingdom of Nri was unusual in the history of world government in that its leader exercised no military power over his subjects. The kingdom existed as a sphere of religious and political influence over a third of Igboland and was administered by