The location of Igbomina in the middle belt of Nigeria and as a frontier Yoruba region opened it to the influence from powerful, competing states of Oyo, Nupe, Fulani, and Ibadan. The expansion of the Oyo Empire in the 16th century, which was accompanied by the large migration of Yoruba northward, led to frequent military aggression on the northern boundary with the Nupe. This paper examines military aggressions in the Igbomina area of north central Yorubaland. Military threat or warfare had initiated various responses in Igbomina, as evident in community aggregation, building of fortifications, production of weapons, and settlement abandonment. The high level of military aggression in Igbomina had also acted as an instrument of socio-political changes in the area as seen in increased centralized control and hierarchy. Oral-historical sources, archaeological survey and excavation form the core of this examination of military aggression in Igbomina.