Well, we do need to remember that Achebe presents us with a fictional account of the history of Okonkwo's tribe and the way in which the white European colonists arrived, often hand-in-hand with missionaries and religion. This means that Achebe presents us with a series of fictional characters and events that perhaps never actually occurred. In particular, the character of Okonkwo is not a historical character and is a character that Achebe creates to characterise the death of traditional culture and the way in which Africa found itself so unmanned and helpless in the face of colonism.
However, apart from the obvious elements of fiction in this novel, Achebe works very hard to present the realities of tribal life before the era of colonialism. Throughout the novel, but in the first half in particular, he devotes much time and attention to exploring and describing cultural aspects of this tribe that present us with a group of people that have their own cohesive world view, although it is obviously a world view that is very different from our own world view. Achebe deliberately paints a picture of an African tribal group and shows us that they are not uncivilised brutes without justice or order, and indeed that their system managed to operate to sustain and unify a group of people over thousands of years, well before the colonialists came.