I would argue that the major event that occurs is the arrival of white man to Okonkwo's tribe and the way that slowly but surely, it begins to insidiously change his society so radically and topple indigenous beliefs and values. This is of course the major point of the novel, as Achebe seeks to present colonial history from the perspective of those being colonised, dispensing with the narratives that of native ignorance and heathenism that seemed to justify the colonial endeavour on the part of the whites.
It is the arrival of the European colonists that also gives the novel its title, which is a direct allusion to "The Second Coming" by Yeats. The way in which centuries of stable cultural identity is displaced in a matter of a few years by white settlers forces us to look at the image of a world losing control and the absolute collapse of tribal society that is summoned up by the words of this poem by Yeats. The image of the falcon turning further and further away from the falconer speaks of the sense of imminent disaster that we see featured in this book as Okonkwo's world changes forever.
There are several significant events that take place throughout the course of the novel. One of the most important events that happens in the story involves Ikemefuna's death. After Ikemefuna has lived with Okonkwo and his family for three years, the Oracle instructs the clan to sacrificially kill Ikemefuna. Although Okonkwo is specifically told not to have any part in Ikemefuna's death, Okonkwo ends up killing him because he does not want to appear weak. This incident reveals Okonkwo's insecurity and fear. Okonkwo's decision to kill Ikemefuna out of fear also foreshadows his own suicide later on in the novel.
In Chapter 24, Okonkwo makes the fatal decision to kill one of the district commissioner's messengers. Okonkwo hates the white colonists and realizes that Umuofia will not go to war against them. Out of fear and hate, Okonkwo murders the white messenger. Rather than be arrested and tried in the white court, Okonkwo hangs himself. Okonkwo's decision to commit suicide is considered a disgrace among his tribe, and he dies without any honor, like his father.