After the defeat of Ilorin by Ibadan in 1840, rivalry between Ibadan and Ijaye grew. In Ibadan the population had increased to over 60,000 by 1851. The Oyo Yoruba had come to dominate the political life of the town, and a political system gradually evolved which was well suited to military expansion (Awe,1967). There was no Oba, and chiefships were not hereditary. The chiefs were organised into four lines: the civil chiefs, led by the Bale; the military chiefs in two lines, headed by the Balogun and the Seriki; and the women chiefs led by the Iyalode. Within each of these lines the titles were ranked, and each chief moved up a rank as those above him died or were killed in battle. The bottom ranks were filled by magaji, the elected leaders of the Ibadan descent groups. The most senior title, that of Bale, was usually filled by a Balogun who had proved himself in war. The fact that there was no oba reflected the theoretical suzerainty of the Alafin, though from its foundation Ibadan pursued an independent foreign policy. In the 19th century the military chiefs usually had the greatest authority. Promotion to a title depended on a man's ability to mobilise a following and on military skill. Prestige and wealth came from warfare and the result was an aggressive policy of expansion.
Ijaye was founded about the same time as Ibadan, by refugees from the Ikoyi area, led by Kurunmi, described by Johnson as the 'greatest soldier of his age'. It became an important communications centre, and under strong leadership it prospered. Mann, the CMS missionary, lived in the town in the 1850s, and he provided much first-hand information on it. By this time, Ijaye probably had a population of 40,000 or more. Initially, relations with Ibadan were good, but rivalry between the two gradually developed. An issue for a final confrontation was provided by the death of Alafin Atiba in 1859. He was succeeded by the Aremo Adelu, and Kurunmi refused to recognise the succession. Ijaye and Oyo were already at loggerheads over the control of the Upper Ogun towns around Saki. In any case, Ibadan sided with the new Alafin and war broke out. Kurunmi died in 1861, before the final capture and destruction of his town.