The obvious answer to this question is that the arrival of Christian missionaries instigates "things falling apart" in Umuofia. Indeed, the Christians change the culture of the village and accelerate divides between clanspeople.
The conditions for "falling apart" are already in place, to some extent, when the Christians arrive, though. For example, the local religion's treatment of outcasts, twins, and other personae non grata makes Christianity's acceptance and apparent equality more appealing to those who feel like outsiders. The traditions of leaving twins in the forest or allowing those sick with certain illnesses to lie outside and die beyond the village limits seems excessively harsh to the missionaries. People frustrated with the clan's traditions are sympathetic to the missionaries and open-minded to a new way of thinking and behaving.
Along with the village itself, Okonkwo , the tragic hero, falls apart. As with any tragic hero, his fall is caused by his tragic flaw, or hamartia....
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