The central conflict is man vs. society. Achebe used his novels to discuss the influence of colonization on the peoples of Africa, but also as social commentaries on the nature of colonization and the after effects.
The central conflict (from which many others stem) is the "clash between the civilization of the Igbo and the British bringing colonialism and their Christian religion to West Africa."
You can learn more about Achebe's novel (themes, characters, etc.) by following the link below.