How is Okonkwo's downfall caused, in part, by circumstances beyond his control?

In Chinua Achebe's novel Things Fall Apart, the protagonist Okonkwo experiences a fall from grace caused, in part, by his own means. However, there are factors that contribute to his downfall that are completely beyond his control: namely, imperialism, or colonialism. 

During the context of the novel, Nigerians are for the first time interacting with white people. This historical period was characterized by white people going into non-European cultures and appropriating their land, resources, people, and culture. Europeans acted in an ethnocentric manner, meaning they believed that their way of life was inherently superior to all others; this affected their perspectives on others' religions, traditions, cultural norms and mores, relationships, political structures, economies, and more. 

In part, Okonkwo's downfall is directly related to the rise of white imperialism, which he was powerless to prevent and was directly affected by.