An old kind of warrant chief, from The Nigeria Handbook, 1936.

As a largely acephalous people*, the British colonial government found it difficult to incorporate Igbo communities into the imperialist system of indirect rule, in response to this the British set up a system in which Igbo communities elected one of their members as a "Warrant Chief" who would be given a 'warrant' to act as a representative of the colonial administration in their community under the 'native court' system, including the responsibility of collecting tax. The system was not well understood by the Igbo who came to decisions via consensus which often included a debate; with the misunderstanding of what the Warrant Chief's powers entailed, many communities elected individuals in their communities who hadn't necessarily been significant in terms of leadership, other communities elected lineage heads and other leaders. The system became widely abused and many Warrant Chiefs amassed wealth through their positions and quickly became despised by their communities.

It was the abuse and threat of taxes by the British through Warrant Chiefs that sparked the Aba women's movement of 1929 in which women led protests and demonstrations and skirmished against Warrant Chiefs and the colonial administration. After 1929, Warrant Chiefs were removed from power, although some of them and their descendants became big men and took up chieftaincy titles. The British then devised Native Authority Councils in which they tried to 'prefect' indirect rule by matching it with what their intelligence reports had told them were traditional organisational structures, this included a council of elders and an elite leading the community, however, women's roles in traditional Igbo organisation were not recognised in this system. After the exit of the British at independence, many of these leaders and their descendants sought traditional legitimisation in many ways, one such example of this was the changing of their official title of 'Chief' to 'Igwe' and 'Eze' in order to further root their status in tradition, it's not unusual to see monarchies in Igboland that were started by an individual originally and officially referred to as 'Chief' but whose descendants are titled 'Eze' for example.

[*Excluding some communities and excluding priest kings and the system of lineage heads who are (originally) fundamentally priests of the lineage, e.g Okpara, Di Okpara, Dede, etc, and senior communities and households.]

More information: Axel Harneit-Sievers (1998). Igbo 'Traditional Rulers': Chieftaincy and the State in Southeastern Nigeria. Africa Spectrum Vol. 33, No. 1. http://www.jstor.org/stable/40174764